Don't dllexport reference temporaries

[hans]

Summary

Even if the reference itself is dllexport, the temporary should not be -- in fact, we're already giving it internal linkage, so dllexporting it is not just wasteful, but will fail to link, as in the example below:

$ cat /tmp/a.cc
void _DllMainCRTStartup() {}
const int __declspec(dllexport) &foo = 42;

$ clang-cl -fuse-ld=lld /tmp/a.cc /Zl /link /dll /out:a.dll
lld-link: error: <root>: undefined symbol: int const &foo::$RT1

[mstorsjo]

LGTM, seems reasonable!

clang/lib/CodeGen/CodeGenModule.cpp

if (Linkage == llvm::GlobalVariable::ExternalLinkage) {
     }
   }
   auto TargetAS = getContext().getTargetAddressSpace(AddrSpace);
   auto *GV = new llvm::GlobalVariable(
       getModule(), Type, Constant, Linkage, InitialValue, Name.c_str(),
       /*InsertBefore=*/nullptr, llvm::GlobalVariable::NotThreadLocal, TargetAS);
   if (emitter) emitter->finalize(GV);
   setGVProperties(GV, VD);
+  if (GV->getDLLStorageClass() == llvm::GlobalVariable::DLLExportStorageClass)
+    // The reference temporary should never be dllexport.
+    GV->setDLLStorageClass(llvm::GlobalVariable::DefaultStorageClass);
   GV->setAlignment(Align.getAsAlign());
   if (supportsCOMDAT() && GV->isWeakForLinker())
     GV->setComdat(TheModule.getOrInsertComdat(GV->getName()));
   if (VD->getTLSKind())
     setTLSMode(GV, *VD);
   llvm::Constant *CV = GV;
   if (AddrSpace != LangAS::Default)
     CV = getTargetCodeGenInfo().performAddrSpaceCast(

clang/test/CodeGenCXX/reference-temporary-ms.cpp

+// RUN: %clang_cc1 -emit-llvm %s -o - -triple=i386-pc-win32 -fms-extensions | FileCheck %s
+
+const int __declspec(dllexport) &Exported = 42;
+
+// The reference temporary shouldn't be dllexport, even if the reference is.
+// CHECK: @"?$RT1@Exported@@3ABHB" = internal constant i32 42
+
+// CHECK: @"?Exported@@3ABHB" = dso_local dllexport constant i32* @"?$RT1@Exported@@3ABHB"