[Metal] Add support for -Fre for Metal (microsoft#8159)

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

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

Author: llvm-beanz

tools/clang/test/DXC/metal.test

@@ -2,6 +2,9 @@
 
 // Metal libraries are LLVM bitcode. This check inspects the magic number from
 // the metal library output.
-// RUN: %dxc %S/Inputs/smoke.hlsl  /T ps_6_0 -metal -Fo Tmp.metal
+// RUN: %dxc %S/Inputs/smoke.hlsl  /T ps_6_0 -metal -Fo Tmp.metal -Fre Tmp.json
 // RUN: head -c 4 Tmp.metal | FileCheck -check-prefix=MTL %s
 // MTL: {{^MTLB}}
+
+// RUN: cat Tmp.json | FileCheck --check-prefix=FRE %s
+// FRE: "EntryPoint":"main"

tools/clang/tools/dxcompiler/dxcompilerobj.cpp

@@ -1085,7 +1085,10 @@ class DxcCompiler : public IDxcCompiler3,
               }
             }
 
-            if (pReflectionStream && pReflectionStream->GetPtrSize()) {
+            // Skip reflection generation here when targeting metal, this is
+            // handled below.
+            if (!opts.GenMetal && pReflectionStream &&
+                pReflectionStream->GetPtrSize()) {
               CComPtr<IDxcBlob> pReflection;
               IFT(pReflectionStream->QueryInterface(&pReflection));
               IFT(pResult->SetOutputObject(DXC_OUT_REFLECTION, pReflection));
@@ -1173,6 +1176,16 @@ class DxcCompiler : public IDxcCompiler3,
                 std::unique_ptr<uint8_t[]>(new uint8_t[MetalLibSize]);
             IRMetalLibGetBytecode(MetalLib, MetalLibBytes.get());
 
+            if (!opts.OutputReflectionFile.empty()) {
+              IRShaderReflection *Reflection = IRShaderReflectionCreate();
+              IRObjectGetReflection(AIR, Stage, Reflection);
+              const char *RawJSON =
+                  IRShaderReflectionCopyJSONString(Reflection);
+              IFT(pResult->SetOutputString(DXC_OUT_REFLECTION, RawJSON,
+                                           strlen(RawJSON)));
+              IRShaderReflectionDestroy(Reflection);
+            }
+
             // Store the metallib to custom format or disk, or use to create a
             // MTLLibrary.