[Driver] Make -static-libgcc imply static libunwind

In the GNU toolchain, `-static-libgcc` implies that the unwindlib will
be linked statically. However, when `--unwindlib=libunwind`, this flag is
ignored, and a bare `-lunwind` is added to the linker args.  Unfortunately,
this means that if both `libunwind.so`, and `libunwind.a` are present
in the library path, `libunwind.so` will be chosen in all cases where
`-static` is not set.

This change makes `-static-libgcc` affect the `-l` flag produced by
`--unwindlib=libunwind`. After this patch, providing
`-static-libgcc --unwindlib=libunwind` will cause the driver to explicitly
emit `-l:libunwind.a` to statically link libunwind. For all other cases
it will emit `-l:libunwind.so` matching current behavior with a more
explicit link line.

https://reviews.llvm.org/D70416

Author: joshkunz

clang/lib/Driver/ToolChains/CommonArgs.cpp

@@ -1209,7 +1209,10 @@ static void AddUnwindLibrary(const ToolChain &TC, const Driver &D,
     break;
   }
   case ToolChain::UNW_CompilerRT:
-    CmdArgs.push_back("-lunwind");
+    if (LGT == LibGccType::StaticLibGcc)
+      CmdArgs.push_back("-l:libunwind.a");
+    else
+      CmdArgs.push_back("-l:libunwind.so");
     break;
   }
 

clang/test/Driver/compiler-rt-unwind.c

@@ -13,7 +13,15 @@
 // RUN:     --gcc-toolchain="" \
 // RUN:   | FileCheck --check-prefix=RTLIB-GCC-UNWINDLIB-COMPILER-RT %s
 // RTLIB-GCC-UNWINDLIB-COMPILER-RT: "{{.*}}lgcc"
-// RTLIB-GCC-UNWINDLIB-COMPILER-RT: "{{.*}}lunwind"
+// RTLIB-GCC-UNWINDLIB-COMPILER-RT: "{{.*}}l:libunwind.so"
+//
+// RUN: %clang -no-canonical-prefixes %s -### -o %t.o 2>&1 \
+// RUN:     --target=x86_64-unknown-linux -rtlib=libgcc --unwindlib=libunwind \
+// RUN:     -static-libgcc \
+// RUN:     --gcc-toolchain="" \
+// RUN:   | FileCheck --check-prefix=RTLIB-GCC-STATIC-UNWINDLIB-COMPILER-RT %s
+// RTLIB-GCC-STATIC-UNWINDLIB-COMPILER-RT: "{{.*}}lgcc"
+// RTLIB-GCC-STATIC-UNWINDLIB-COMPILER-RT: "{{.*}}l:libunwind.a"
 //
 // RUN: %clang -no-canonical-prefixes %s -### -o %t.o 2>&1   \
 // RUN:     --target=x86_64-unknown-linux -rtlib=compiler-rt \