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
diff --git a/clang/lib/CodeGen/CGVTables.cpp b/clang/lib/CodeGen/CGVTables.cpp
index 2e8471e..372db7a 100644
--- a/clang/lib/CodeGen/CGVTables.cpp
+++ b/clang/lib/CodeGen/CGVTables.cpp
@@ -743,7 +743,7 @@
     return DiscardableODRLinkage;
 
   case TSK_ExplicitInstantiationDeclaration:
-    llvm_unreachable("Should not have been asked to emit this");
+    return llvm::GlobalVariable::ExternalLinkage;
 
   case TSK_ExplicitInstantiationDefinition:
     return NonDiscardableODRLinkage;