[Multilib] Add -fmultilib-flag command-line option

This option is passed through to the multilib system. It is then used in
conjunction with other options to select the proper library variant.

Details about this change can be found in this thread:
https://discourse.llvm.org/t/rfc-multilib-custom-flags/81058

Author: vhscampos

clang/include/clang/Driver/Options.td

@@ -5658,6 +5658,8 @@ def print_multi_directory : Flag<["-", "--"], "print-multi-directory">;
 def print_multi_lib : Flag<["-", "--"], "print-multi-lib">;
 def print_multi_flags : Flag<["-", "--"], "print-multi-flags-experimental">,
   HelpText<"Print the flags used for selecting multilibs (experimental)">;
+def fmultilib_flag : Joined<["-", "--"], "fmultilib-flag=">,
+  Visibility<[ClangOption]>;
 def print_multi_os_directory : Flag<["-", "--"], "print-multi-os-directory">,
   Flags<[Unsupported]>;
 def print_target_triple : Flag<["-", "--"], "print-target-triple">,

clang/lib/Driver/ToolChain.cpp

@@ -196,6 +196,16 @@ bool ToolChain::defaultToIEEELongDouble() const {
   return PPC_LINUX_DEFAULT_IEEELONGDOUBLE && getTriple().isOSLinux();
 }
 
+static void
+processARMAArch64MultilibCustomFlags(Multilib::flags_list &List,
+                                     const llvm::opt::ArgList &Args) {
+  for (const Arg *MultilibFlagArg :
+       Args.filtered(options::OPT_fmultilib_flag)) {
+    List.push_back(MultilibFlagArg->getAsString(Args));
+    MultilibFlagArg->claim();
+  }
+}
+
 static void getAArch64MultilibFlags(const Driver &D,
                                           const llvm::Triple &Triple,
                                           const llvm::opt::ArgList &Args,
@@ -227,6 +237,8 @@ static void getAArch64MultilibFlags(const Driver &D,
   if (BranchProtectionArg) {
     Result.push_back(BranchProtectionArg->getAsString(Args));
   }
+
+  processARMAArch64MultilibCustomFlags(Result, Args);
 }
 
 static void getARMMultilibFlags(const Driver &D,
@@ -280,6 +292,8 @@ static void getARMMultilibFlags(const Driver &D,
   if (BranchProtectionArg) {
     Result.push_back(BranchProtectionArg->getAsString(Args));
   }
+
+  processARMAArch64MultilibCustomFlags(Result, Args);
 }
 
 static void getRISCVMultilibFlags(const Driver &D, const llvm::Triple &Triple,

clang/test/Driver/print-multi-selection-flags.c

@@ -77,3 +77,8 @@
 // CHECK-RV32E-ORDER: --target=riscv32-unknown-none-elf
 // CHECK-RV32E-ORDER: -mabi=ilp32e
 // CHECK-RV32E-ORDER: -march=rv32e{{[0-9]+p[0-9]+}}_c{{[0-9]+p[0-9]+}}_zicsr{{[0-9]+p[0-9]+}}
+
+// RUN: %clang -print-multi-flags-experimental --target=armv8m.main-none-eabi -fmultilib-flag=foo -fmultilib-flag=bar | FileCheck --check-prefix=CHECK-MULTILIB-CUSTOM-FLAG %s
+// CHECK-MULTILIB-CUSTOM-FLAG: --target=thumbv8m.main-unknown-none-eabi
+// CHECK-MULTILIB-CUSTOM-FLAG-DAG: -fmultilib-flag=foo
+// CHECK-MULTILIB-CUSTOM-FLAG-DAG: -fmultilib-flag=bar