[clang] Introduce a new UEFI target predefine. (#111719)

Add a `__UEFI__` predefine for UEFI target.

Co-authored-by: RossComputerGuy

Author: Prabhuk

clang/lib/Basic/Targets/OSTargets.h

@@ -790,7 +790,9 @@ template <typename Target>
 class LLVM_LIBRARY_VISIBILITY UEFITargetInfo : public OSTargetInfo<Target> {
 protected:
   void getOSDefines(const LangOptions &Opts, const llvm::Triple &Triple,
-                    MacroBuilder &Builder) const override {}
+                    MacroBuilder &Builder) const override {
+    Builder.defineMacro("__UEFI__");
+  }
 
 public:
   UEFITargetInfo(const llvm::Triple &Triple, const TargetOptions &Opts)

clang/test/Driver/uefi-defines.c

@@ -0,0 +1,6 @@
+// RUN: %clang -target x86_64-unknown-uefi %s -emit-llvm -S -c -o - | FileCheck %s
+
+// CHECK: __UEFI__defined
+#ifdef __UEFI__
+void __UEFI__defined() {}
+#endif

clang/test/Preprocessor/init.c

@@ -2742,3 +2742,7 @@
 // RISCV64-LINUX: #define __unix__ 1
 // RISCV64-LINUX: #define linux 1
 // RISCV64-LINUX: #define unix 1
+
+// RUN: %clang_cc1 -dM -triple=x86_64-uefi -E /dev/null | FileCheck -match-full-lines -check-prefix UEFI %s
+
+// UEFI: #define __UEFI__ 1