[Driver] Improve error when a compiler-rt library is not found

BSD, Linux, and z/OS enable `LLVM_ENABLE_PER_TARGET_RUNTIME_DIR` by default.
When a compiler-rt library is not found, we currently report an
incorrect filename `libclang_rt.XXX-$arch.a`
```
% /tmp/Debug/bin/clang++ a.cc -fsanitize=address -o a
ld.lld: error: cannot open /tmp/Debug/lib/clang/19/lib/linux/libclang_rt.asan-x86_64.a: No such file or directory
clang++: error: linker command failed with exit code 1 (use -v to see invocation)
```

With this change, we will correctly report:
```
% /tmp/Debug/bin/clang++ a.cc -fsanitize=address -o a
ld.lld: error: cannot open /tmp/Debug/lib/clang/19/lib/x86_64-unknown-linux-gnu/libclang_rt.asan.a: No such file or directory
clang++: error: linker command failed with exit code 1 (use -v to see invocation)
```

Link: https://discourse.llvm.org/t/runtime-directory-fallback/76860

Author: MaskRay

clang/lib/Driver/ToolChain.cpp

@@ -656,19 +656,29 @@ std::string ToolChain::getCompilerRT(const ArgList &Args, StringRef Component,
   // Check for runtime files in the new layout without the architecture first.
   std::string CRTBasename =
       buildCompilerRTBasename(Args, Component, Type, /*AddArch=*/false);
+  SmallString<128> Path;
   for (const auto &LibPath : getLibraryPaths()) {
     SmallString<128> P(LibPath);
     llvm::sys::path::append(P, CRTBasename);
     if (getVFS().exists(P))
       return std::string(P);
+    if (Path.empty())
+      Path = P;
   }
+  if (getTriple().isOSAIX())
+    Path.clear();
 
-  // Fall back to the old expected compiler-rt name if the new one does not
-  // exist.
+  // Check the filename for the old layout if the new one does not exist.
   CRTBasename =
       buildCompilerRTBasename(Args, Component, Type, /*AddArch=*/true);
-  SmallString<128> Path(getCompilerRTPath());
-  llvm::sys::path::append(Path, CRTBasename);
+  SmallString<128> OldPath(getCompilerRTPath());
+  llvm::sys::path::append(OldPath, CRTBasename);
+  if (Path.empty() || getVFS().exists(OldPath))
+    return std::string(OldPath);
+
+  // If none is found, use a file name from the new layout, which may get
+  // printed in an error message, aiding users in knowing what Clang is
+  // looking for.
   return std::string(Path);
 }
 

clang/test/Driver/Inputs/resource_dir/lib/linux/libclang_rt.asan-i386.a.syms renamed to clang/test/Driver/Inputs/resource_dir/lib/aarch64-unknown-linux/libclang_rt.hwasan.a

@@ -49,4 +49,4 @@
 //
 // Have a stricter check for no-archdir - that the driver doesn't add any
 // subdirectory from the provided resource directory.
-// NO-ARCHDIR-NOT: -L[[FILE_PATH]]/Inputs/resource_dir
+// NO-ARCHDIR-NOT: -L[[FILE_PATH]]/Inputs/resource_dir"

clang/test/Driver/Inputs/resource_dir/lib/linux/libclang_rt.asan-x86_64.a.syms renamed to clang/test/Driver/Inputs/resource_dir/lib/aarch64-unknown-linux/libclang_rt.hwasan.a.syms

@@ -3,7 +3,7 @@
 // RUN:     -resource-dir=%S/Inputs/resource_dir_with_arch_subdir \
 // RUN:     -rtlib=compiler-rt -### %s 2>&1 \
 // RUN:   | FileCheck %s -check-prefix ARM-EABI
-// ARM-EABI: "-lclang_rt.builtins-arm"
+// ARM-EABI: "-lclang_rt.builtins"
 
 // RUN: %clang -target arm-linux-gnueabi \
 // RUN:     --sysroot=%S/Inputs/resource_dir_with_arch_subdir \

clang/test/Driver/Inputs/resource_dir/lib/linux/libclang_rt.hwasan-aarch64.a.syms renamed to clang/test/Driver/Inputs/resource_dir/lib/i386-unknown-linux/libclang_rt.asan.a

@@ -18,5 +18,5 @@
 // CHECK-V6M-C-SAME: "-x" "c++" "{{.*}}baremetal-sysroot.cpp"
 // CHECK-V6M-C-NEXT: "{{[^"]*}}ld{{(\.(lld|bfd|gold))?}}{{(\.exe)?}}" "{{.*}}.o" "-Bstatic"
 // CHECK-V6M-C-SAME: "-L{{.*}}/baremetal_default_sysroot{{[/\\]+}}bin{{[/\\]+}}..{{[/\\]+}}lib{{[/\\]+}}clang-runtimes{{[/\\]+}}armv6m-none-eabi{{[/\\]+}}lib"
-// CHECK-V6M-C-SAME: "-lc" "-lm" "-lclang_rt.builtins-armv6m"
+// CHECK-V6M-C-SAME: "-lc" "-lm" "-lclang_rt.builtins"
 // CHECK-V6M-C-SAME: "-o" "{{.*}}.o"

clang/test/Driver/Inputs/resource_dir/lib/linux/libclang_rt.hwasan-x86_64.a.syms renamed to clang/test/Driver/Inputs/resource_dir/lib/i386-unknown-linux/libclang_rt.asan.a.syms

@@ -52,7 +52,7 @@
 // RUN:     --target=x86_64-unknown-linux -rtlib=compiler-rt \
 // RUN:     --gcc-toolchain="" -resource-dir=%S/Inputs/resource_dir \
 // RUN:   | FileCheck --check-prefix=RTLIB-COMPILER-RT %s
-// RTLIB-COMPILER-RT: "{{.*}}libclang_rt.builtins-x86_64.a"
+// RTLIB-COMPILER-RT: "{{.*}}libclang_rt.builtins.a"
 //
 // RUN: %clang -### %s 2>&1 \
 // RUN:     --target=x86_64-unknown-linux -rtlib=compiler-rt --unwindlib=libunwind \
@@ -62,7 +62,7 @@
 // RUN:     --target=x86_64-unknown-linux -rtlib=compiler-rt --unwindlib=libunwind \
 // RUN:     --gcc-toolchain="" -resource-dir=%S/Inputs/resource_dir \
 // RUN:   | FileCheck --check-prefix=RTLIB-COMPILER-RT-UNWINDLIB-COMPILER-RT %s
-// RTLIB-COMPILER-RT-UNWINDLIB-COMPILER-RT: "{{.*}}libclang_rt.builtins-x86_64.a"
+// RTLIB-COMPILER-RT-UNWINDLIB-COMPILER-RT: "{{.*}}libclang_rt.builtins.a"
 // RTLIB-COMPILER-RT-UNWINDLIB-COMPILER-RT-SAME: "--as-needed"
 // RTLIB-COMPILER-RT-UNWINDLIB-COMPILER-RT-SAME: "-lunwind"
 // RTLIB-COMPILER-RT-UNWINDLIB-COMPILER-RT-SAME: "--no-as-needed"
@@ -71,14 +71,14 @@
 // RUN:     --target=x86_64-unknown-linux -rtlib=compiler-rt --unwindlib=libgcc \
 // RUN:     --gcc-toolchain="" -resource-dir=%S/Inputs/resource_dir \
 // RUN:   | FileCheck --check-prefix=RTLIB-COMPILER-RT-UNWINDLIB-GCC %s
-// RTLIB-COMPILER-RT-UNWINDLIB-GCC: "{{.*}}libclang_rt.builtins-x86_64.a"
+// RTLIB-COMPILER-RT-UNWINDLIB-GCC: "{{.*}}libclang_rt.builtins.a"
 // RTLIB-COMPILER-RT-UNWINDLIB-GCC-SAME: "-lgcc_s"
 //
 // RUN: %clang -### %s 2>&1              \
 // RUN:     --target=x86_64-unknown-linux -rtlib=compiler-rt --unwindlib=libgcc \
 // RUN:     -static --gcc-toolchain="" -resource-dir=%S/Inputs/resource_dir \
 // RUN:   | FileCheck --check-prefix=RTLIB-COMPILER-RT-UNWINDLIB-GCC-STATIC %s
-// RTLIB-COMPILER-RT-UNWINDLIB-GCC-STATIC: "{{.*}}libclang_rt.builtins-x86_64.a"
+// RTLIB-COMPILER-RT-UNWINDLIB-GCC-STATIC: "{{.*}}libclang_rt.builtins.a"
 // RTLIB-COMPILER-RT-UNWINDLIB-GCC-STATIC-SAME: "-lgcc_eh"
 //
 // RUN: not %clang %s 2> %t.err              \

clang/test/Driver/Inputs/resource_dir/lib/linux/libclang_rt.msan-x86_64.a.syms renamed to clang/test/Driver/Inputs/resource_dir/lib/i686-unknown-linux/clang_rt.crtbegin.o

@@ -13,7 +13,7 @@
 // RUN:   | FileCheck --check-prefix=CHECK-LINUX-I386 %s
 //
 // CHECK-LINUX-I386-NOT: "-u__llvm_profile_runtime"
-// CHECK-LINUX-I386: /Inputs/resource_dir{{/|\\\\}}lib{{/|\\\\}}linux{{/|\\\\}}libclang_rt.profile-i386.a"
+// CHECK-LINUX-I386: /Inputs/resource_dir{{/|\\\\}}lib{{.*}}i386-unknown-linux{{.*}}libclang_rt.profile.a"
 // CHECK-LINUX-I386-NOT: "-u__llvm_profile_runtime"
 // CHECK-LINUX-I386: "-lc"
 //
@@ -24,7 +24,7 @@
 // RUN:   | FileCheck --check-prefix=CHECK-LINUX-X86-64 %s
 //
 // CHECK-LINUX-X86-64: "{{(.*[^-.0-9A-Z_a-z])?}}ld{{(.exe)?}}"
-// CHECK-LINUX-X86-64: "{{.*}}/Inputs/resource_dir{{/|\\\\}}lib{{/|\\\\}}linux{{/|\\\\}}libclang_rt.profile-x86_64.a" {{.*}} "-lc"
+// CHECK-LINUX-X86-64: "{{.*}}/Inputs/resource_dir{{/|\\\\}}lib{{.*}}linux{{.*}}libclang_rt.profile.a" {{.*}} "-lc"
 //
 // RUN: %clang -### %s 2>&1 \
 // RUN:     --target=x86_64-unknown-freebsd --coverage -fuse-ld=ld \

clang/test/Driver/Inputs/resource_dir/lib/linux/libclang_rt.msan_cxx-x86_64.a.syms renamed to clang/test/Driver/Inputs/resource_dir/lib/i686-unknown-linux/clang_rt.crtend.o

@@ -187,7 +187,7 @@
 // CHECK-SCUDO-X86: "-resource-dir" "[[RESOURCE_DIR:[^"]+]]"
 // CHECK-SCUDO-X86: "-fsanitize=safe-stack,scudo"
 // CHECK-SCUDO-X86: "-pie"
-// CHECK-SCUDO-X86: "[[RESOURCE_DIR]]{{/|\\\\}}lib{{/|\\\\}}fuchsia{{/|\\\\}}libclang_rt.scudo_standalone-x86_64.so"
+// CHECK-SCUDO-X86: "[[RESOURCE_DIR]]{{/|\\\\}}lib{{.*}}fuchsia{{.*}}libclang_rt.scudo_standalone.so"
 
 // RUN: %clang -### %s --target=aarch64-unknown-fuchsia \
 // RUN:     -fsanitize=scudo 2>&1 \
@@ -197,7 +197,7 @@
 // CHECK-SCUDO-AARCH64: "-resource-dir" "[[RESOURCE_DIR:[^"]+]]"
 // CHECK-SCUDO-AARCH64: "-fsanitize=shadow-call-stack,scudo"
 // CHECK-SCUDO-AARCH64: "-pie"
-// CHECK-SCUDO-AARCH64: "[[RESOURCE_DIR]]{{/|\\\\}}lib{{/|\\\\}}fuchsia{{/|\\\\}}libclang_rt.scudo_standalone-aarch64.so"
+// CHECK-SCUDO-AARCH64: "[[RESOURCE_DIR]]{{/|\\\\}}lib{{.*}}fuchsia{{.*}}libclang_rt.scudo_standalone.so"
 
 // RUN: %clang -### %s --target=x86_64-unknown-fuchsia \
 // RUN:     -fsanitize=scudo -fPIC -shared 2>&1 \
@@ -206,7 +206,7 @@
 // RUN:     | FileCheck %s -check-prefix=CHECK-SCUDO-SHARED
 // CHECK-SCUDO-SHARED: "-resource-dir" "[[RESOURCE_DIR:[^"]+]]"
 // CHECK-SCUDO-SHARED: "-fsanitize=safe-stack,scudo"
-// CHECK-SCUDO-SHARED: "[[RESOURCE_DIR]]{{/|\\\\}}lib{{/|\\\\}}fuchsia{{/|\\\\}}libclang_rt.scudo_standalone-x86_64.so"
+// CHECK-SCUDO-SHARED: "[[RESOURCE_DIR]]{{/|\\\\}}lib{{.*}}fuchsia{{.*}}libclang_rt.scudo_standalone.so"
 
 // RUN: %clang -### %s --target=aarch64-unknown-fuchsia \
 // RUN:     -fsanitize=leak 2>&1 \

clang/test/Driver/Inputs/resource_dir/lib/linux/libclang_rt.tsan-x86_64.a.syms renamed to clang/test/Driver/Inputs/resource_dir/lib/x86_64-unknown-linux/clang_rt.crtbegin.o

@@ -7,7 +7,7 @@
 // RUN:   | FileCheck --check-prefix=CHECK-LINUX-I386 %s
 //
 // CHECK-LINUX-I386: "{{(.*[^-.0-9A-Z_a-z])?}}ld{{(.exe)?}}"
-// CHECK-LINUX-I386: "{{.*}}/Inputs/resource_dir{{/|\\\\}}lib{{/|\\\\}}linux{{/|\\\\}}libclang_rt.profile-i386.a" {{.*}} "-lc"
+// CHECK-LINUX-I386: "{{.*}}/Inputs/resource_dir{{/|\\\\}}lib{{/|\\\\}}i386-unknown-linux{{/|\\\\}}libclang_rt.profile.a" {{.*}} "-lc"
 //
 // RUN: %clang -### %s 2>&1 \
 // RUN:     --target=x86_64-unknown-linux -fprofile-instr-generate -fuse-ld=ld \
@@ -16,7 +16,7 @@
 // RUN:   | FileCheck --check-prefix=CHECK-LINUX-X86-64 %s
 //
 // CHECK-LINUX-X86-64: "{{(.*[^-.0-9A-Z_a-z])?}}ld{{(.exe)?}}"
-// CHECK-LINUX-X86-64: "{{.*}}/Inputs/resource_dir{{/|\\\\}}lib{{/|\\\\}}linux{{/|\\\\}}libclang_rt.profile-x86_64.a" {{.*}} "-lc"
+// CHECK-LINUX-X86-64: "{{.*}}/Inputs/resource_dir{{/|\\\\}}lib{{.*}}linux{{.*}}libclang_rt.profile.a" {{.*}} "-lc"
 //
 // RUN: %clang -### %s 2>&1 \
 // RUN:     --target=x86_64-unknown-linux -fprofile-instr-generate -nostdlib -fuse-ld=ld \
@@ -25,7 +25,7 @@
 // RUN:   | FileCheck --check-prefix=CHECK-LINUX-NOSTDLIB-X86-64 %s
 //
 // CHECK-LINUX-NOSTDLIB-X86-64: "{{(.*[^-.0-9A-Z_a-z])?}}ld{{(.exe)?}}"
-// CHECK-LINUX-NOSTDLIB-X86-64: "{{.*}}/Inputs/resource_dir{{/|\\\\}}lib{{/|\\\\}}linux{{/|\\\\}}libclang_rt.profile-x86_64.a"
+// CHECK-LINUX-NOSTDLIB-X86-64: "{{.*}}/Inputs/resource_dir{{/|\\\\}}lib{{.*}}linux{{.*}}libclang_rt.profile.a"
 //
 // RUN: %clang -### %s 2>&1 \
 // RUN:     --target=x86_64-unknown-freebsd -fprofile-instr-generate -fuse-ld=ld \
@@ -62,7 +62,7 @@
 // RUN:   | FileCheck --check-prefix=CHECK-LINUX-I386-SHARED %s
 //
 // CHECK-LINUX-I386-SHARED: "{{(.*[^-.0-9A-Z_a-z])?}}ld{{(.exe)?}}"
-// CHECK-LINUX-I386-SHARED: "{{.*}}/Inputs/resource_dir{{/|\\\\}}lib{{/|\\\\}}linux{{/|\\\\}}libclang_rt.profile-i386.a" {{.*}} "-lc"
+// CHECK-LINUX-I386-SHARED: "{{.*}}/Inputs/resource_dir{{/|\\\\}}lib{{.*}}i386-unknown-linux{{.*}}libclang_rt.profile.a" {{.*}} "-lc"
 //
 // RUN: %clang -### %s 2>&1 \
 // RUN:     -shared \
@@ -72,7 +72,7 @@
 // RUN:   | FileCheck --check-prefix=CHECK-LINUX-X86-64-SHARED %s
 //
 // CHECK-LINUX-X86-64-SHARED: "{{(.*[^-.0-9A-Z_a-z])?}}ld{{(.exe)?}}"
-// CHECK-LINUX-X86-64-SHARED: "{{.*}}/Inputs/resource_dir{{/|\\\\}}lib{{/|\\\\}}linux{{/|\\\\}}libclang_rt.profile-x86_64.a" {{.*}} "-lc"
+// CHECK-LINUX-X86-64-SHARED: "{{.*}}/Inputs/resource_dir{{/|\\\\}}lib{{.*}}linux{{.*}}libclang_rt.profile.a" {{.*}} "-lc"
 //
 // RUN: %clang -### %s 2>&1 \
 // RUN:     -shared \

clang/test/Driver/Inputs/resource_dir/lib/linux/libclang_rt.tsan_cxx-x86_64.a.syms renamed to clang/test/Driver/Inputs/resource_dir/lib/x86_64-unknown-linux/clang_rt.crtend.o

@@ -61,15 +61,15 @@
 // CHECK-LD-RT: "--eh-frame-hdr"
 // CHECK-LD-RT: "-m" "elf_x86_64"
 // CHECK-LD-RT: "-dynamic-linker"
-// CHECK-LD-RT: "[[RESDIR]]{{/|\\\\}}lib{{/|\\\\}}linux{{/|\\\\}}clang_rt.crtbegin-x86_64.o"
+// CHECK-LD-RT: "[[RESDIR]]{{/|\\\\}}lib{{/|\\\\}}x86_64-unknown-linux{{/|\\\\}}clang_rt.crtbegin.o"
 // CHECK-LD-RT: "-L[[SYSROOT]]/usr/lib/gcc/x86_64-unknown-linux/10.2.0"
 // CHECK-LD-RT: "-L[[SYSROOT]]/usr/lib/gcc/x86_64-unknown-linux/10.2.0/../../../../x86_64-unknown-linux/lib"
 // CHECK-LD-RT: "-L[[SYSROOT]]/lib"
 // CHECK-LD-RT: "-L[[SYSROOT]]/usr/lib"
-// CHECK-LD-RT: libclang_rt.builtins-x86_64.a"
+// CHECK-LD-RT: libclang_rt.builtins.a"
 // CHECK-LD-RT: "-lc"
-// CHECK-LD-RT: libclang_rt.builtins-x86_64.a"
-// CHECK-LD-RT: "[[RESDIR]]{{/|\\\\}}lib{{/|\\\\}}linux{{/|\\\\}}clang_rt.crtend-x86_64.o"
+// CHECK-LD-RT: libclang_rt.builtins.a"
+// CHECK-LD-RT: "[[RESDIR]]{{/|\\\\}}lib{{/|\\\\}}x86_64-unknown-linux{{/|\\\\}}clang_rt.crtend.o"
 //
 // RUN: %clang -### %s -no-pie 2>&1 \
 // RUN:     --target=i686-unknown-linux \
@@ -84,15 +84,15 @@
 // CHECK-LD-RT-I686: "--eh-frame-hdr"
 // CHECK-LD-RT-I686: "-m" "elf_i386"
 // CHECK-LD-RT-I686: "-dynamic-linker"
-// CHECK-LD-RT-I686: "[[RESDIR]]{{/|\\\\}}lib{{/|\\\\}}linux{{/|\\\\}}clang_rt.crtbegin-i386.o"
+// CHECK-LD-RT-I686: "[[RESDIR]]{{/|\\\\}}lib{{/|\\\\}}i686-unknown-linux{{/|\\\\}}clang_rt.crtbegin.o"
 // CHECK-LD-RT-I686: "-L[[SYSROOT]]/usr/lib/gcc/i686-unknown-linux/10.2.0"
 // CHECK-LD-RT-I686: "-L[[SYSROOT]]/usr/lib/gcc/i686-unknown-linux/10.2.0/../../../../i686-unknown-linux/lib"
 // CHECK-LD-RT-I686: "-L[[SYSROOT]]/lib"
 // CHECK-LD-RT-I686: "-L[[SYSROOT]]/usr/lib"
-// CHECK-LD-RT-I686: libclang_rt.builtins-i386.a"
+// CHECK-LD-RT-I686: libclang_rt.builtins.a"
 // CHECK-LD-RT-I686: "-lc"
-// CHECK-LD-RT-I686: libclang_rt.builtins-i386.a"
-// CHECK-LD-RT-I686: "[[RESDIR]]{{/|\\\\}}lib{{/|\\\\}}linux{{/|\\\\}}clang_rt.crtend-i386.o"
+// CHECK-LD-RT-I686: libclang_rt.builtins.a"
+// CHECK-LD-RT-I686: "[[RESDIR]]{{/|\\\\}}lib{{/|\\\\}}i686-unknown-linux{{/|\\\\}}clang_rt.crtend.o"
 //
 // RUN: %clang -### %s -no-pie 2>&1 \
 // RUN:     --target=arm-linux-androideabi \

clang/test/Driver/Inputs/resource_dir/lib/linux/libclang_rt.ubsan-i386.a.syms renamed to clang/test/Driver/Inputs/resource_dir/lib/x86_64-unknown-linux/libclang_rt.asan.a

@@ -55,7 +55,7 @@
 // RUN:     --sysroot=%S/Inputs/resource_dir_with_arch_subdir \
 // RUN:     -resource-dir=%S/Inputs/resource_dir_with_arch_subdir 2>&1 \
 // RUN:   | FileCheck --check-prefix=CHECK-CLANGRT-ARM-BAREMETAL %s
-// CHECK-CLANGRT-ARM-BAREMETAL: libclang_rt.builtins-armv7m.a
+// CHECK-CLANGRT-ARM-BAREMETAL: libclang_rt.builtins.a
 
 // RUN: %clang -rtlib=compiler-rt -print-libgcc-file-name \
 // RUN:     --target=armv7m-vendor-none-eabi \