[Driver][UEFI] Enable Microsoft extensions (llvm#121875)

Set "-fms-extensions" for UEFI targets.

Author: Prabhuk

clang/lib/Driver/ToolChains/Clang.cpp

@@ -5109,6 +5109,7 @@ void Clang::ConstructJob(Compilation &C, const JobAction &JA,
   const llvm::Triple *AuxTriple =
       (IsCuda || IsHIP) ? TC.getAuxTriple() : nullptr;
   bool IsWindowsMSVC = RawTriple.isWindowsMSVCEnvironment();
+  bool IsUEFI = RawTriple.isUEFI();
   bool IsIAMCU = RawTriple.isOSIAMCU();
 
   // Adjust IsWindowsXYZ for CUDA/HIP/SYCL compilations.  Even when compiling in
@@ -7252,7 +7253,7 @@ void Clang::ConstructJob(Compilation &C, const JobAction &JA,
 
   // -fms-extensions=0 is default.
   if (Args.hasFlag(options::OPT_fms_extensions, options::OPT_fno_ms_extensions,
-                   IsWindowsMSVC))
+                   IsWindowsMSVC || IsUEFI))
     CmdArgs.push_back("-fms-extensions");
 
   // -fms-compatibility=0 is default.

clang/test/Driver/uefi-constructed-args.c

@@ -4,6 +4,7 @@
 // CHECK-SAME: "-triple" "x86_64-unknown-uefi"
 // CHECK-SAME: "-mrelocation-model" "pic" "-pic-level" "2"
 // CHECK-SAME: "-mframe-pointer=all"
+// CHECK-SAME: "-fms-extensions"
 // CHECK-NEXT: "-nologo"
 // CHECK-SAME: "-subsystem:efi_application"
 // CHECK-SAME: "-entry:EfiMain"