[Metal] Add support for -Fre for Metal

[llvm-beanz]

This updates the Metal support to enable generating the Metal Shader Converter's reflection JSON file.

Required to fix llvm/offload-test-suite#452.

[github-actions]

✅ With the latest revision this PR passed the C/C++ code formatter.

[Keenuts]

Patching this PR and checking Metal codegen, I have an assert because the reflection output seems to be written before.

Seems like the fix is to do:

--- a/tools/clang/tools/dxcompiler/dxcompilerobj.cpp
+++ b/tools/clang/tools/dxcompiler/dxcompilerobj.cpp
@@ -1085,7 +1085,12 @@ public:
               }
             }
 
-            if (pReflectionStream && pReflectionStream->GetPtrSize()) {
+#ifdef ENABLE_METAL_CODEGEN
+            const bool GenMetal = opts.GenMetal;
+#else
+            const bool GenMetal = false;
+#endif
+            if (!GenMetal && pReflectionStream && pReflectionStream->GetPtrSize()) {
               CComPtr<IDxcBlob> pReflection;
               IFT(pReflectionStream->QueryInterface(&pReflection));
               IFT(pResult->SetOutputObject(DXC_OUT_REFLECTION, pReflection));