MS ABI: Don't try to emit VF/VB-Tables for extern class templates

There will be an explicit template instantiation in another translation
unit which will provide the definition of the VF/VB-Tables.

This fixes PR22932.

llvm-svn: 232680

clang/lib/CodeGen/CGVTables.cpp

@@ -743,7 +743,7 @@ CodeGenModule::getVTableLinkage(const CXXRecordDecl *RD) {
     return DiscardableODRLinkage;
 
   case TSK_ExplicitInstantiationDeclaration:
-    llvm_unreachable("Should not have been asked to emit this");
+    return llvm::GlobalVariable::ExternalLinkage;
 
   case TSK_ExplicitInstantiationDefinition:
     return NonDiscardableODRLinkage;