[CodeGen] Fix invalid llvm.linker.options about pragma detect_mismatch

[yaxunl]

Summary

When a target does not support pragma detect_mismatch, an llvm.linker.options
metadata with an empty entry is created, which causes diagnostic in backend
since backend expects name/value pair in llvm.linker.options entries.

This patch fixes that.

[rjmccall]

Seems reasonable.

clang/lib/CodeGen/CodeGenModule.cpp

 void CodeGenModule::AppendLinkerOptions(StringRef Opts) {
   auto *MDOpts = llvm::MDString::get(getLLVMContext(), Opts);
   LinkerOptionsMetadata.push_back(llvm::MDNode::get(getLLVMContext(), MDOpts));
 }
 
 void CodeGenModule::AddDetectMismatch(StringRef Name, StringRef Value) {
   llvm::SmallString<32> Opt;
   getTargetCodeGenInfo().getDetectMismatchOption(Name, Value, Opt);
+  if (Opt.empty())
+    return;
   auto *MDOpts = llvm::MDString::get(getLLVMContext(), Opt);
   LinkerOptionsMetadata.push_back(llvm::MDNode::get(getLLVMContext(), MDOpts));
 }
 
 void CodeGenModule::AddDependentLib(StringRef Lib) {
   auto &C = getLLVMContext();
   if (getTarget().getTriple().isOSBinFormatELF()) {
       ELFDependentLibraries.push_back(

clang/test/CodeGen/pragma-detect_mismatch.c

-// RUN: %clang_cc1 %s -triple i686-pc-win32 -fms-extensions -emit-llvm -o - | FileCheck %s
-// RUN: %clang_cc1 %s -triple thumbv7-windows -fms-extensions -emit-llvm -o - | FileCheck %s
+// RUN: %clang_cc1 %s -triple i686-pc-win32 -fms-extensions -emit-llvm -o - \
+// RUN:   | FileCheck %s
+// RUN: %clang_cc1 %s -triple thumbv7-windows -fms-extensions -emit-llvm -o - \
+// RUN:   | FileCheck %s
+// RUN: %clang_cc1 %s -triple amdgcn-amd-amdhsa -fms-extensions -emit-llvm -o \
+// RUN:   - | FileCheck -check-prefix=AMD %s
 
 #pragma detect_mismatch("test", "1")
 
 #define BAR "2"
 #pragma detect_mismatch("test2", BAR)
 
 // CHECK: !llvm.linker.options = !{![[test:[0-9]+]], ![[test2:[0-9]+]]}
 // CHECK: ![[test]] = !{!"/FAILIFMISMATCH:\22test=1\22"}
 // CHECK: ![[test2]] = !{!"/FAILIFMISMATCH:\22test2=2\22"}
+// AMD-NOT: !llvm.linker.options