[MS ABI] Correctly handle dllimport'd explicit instantiation declaration w/ vbases

We gave a VBTable dllimport storage class and external linkage while
also providing an initializer.  An initializer is only valid if the
VBTable has available_externally linkage.  Fix this by setting the
linkage to available_externally in situ while generating the
initializer.

This fixes PR26686.

llvm-svn: 261535
diff --git a/clang/lib/CodeGen/MicrosoftCXXABI.cpp b/clang/lib/CodeGen/MicrosoftCXXABI.cpp
index 2567aa9..eecb12c 100644
--- a/clang/lib/CodeGen/MicrosoftCXXABI.cpp
+++ b/clang/lib/CodeGen/MicrosoftCXXABI.cpp
@@ -2039,6 +2039,9 @@
     llvm::ArrayType::get(CGM.IntTy, Offsets.size());
   llvm::Constant *Init = llvm::ConstantArray::get(VBTableType, Offsets);
   GV->setInitializer(Init);
+
+  if (RD->hasAttr<DLLImportAttr>())
+    GV->setLinkage(llvm::GlobalVariable::AvailableExternallyLinkage);
 }
 
 llvm::Value *MicrosoftCXXABI::performThisAdjustment(CodeGenFunction &CGF,