Re-land "[MS] Don't expect vftables to be provided for extern template instantiations"

Reverts r273305 and re-instates r273296.

We needed to fix a bug in Sema::MarkVTableUsed to ensure that operator
delete lookup occurs when the vtable is referenced. We already had a
special case to look up operator delete when dllimport was used, but I
think should really mark virtual destructors referenced any time the
vtable is used.

llvm-svn: 274147
diff --git a/clang/test/CodeGenCXX/microsoft-abi-vbtables.cpp b/clang/test/CodeGenCXX/microsoft-abi-vbtables.cpp
index 9cce6f8..df06894 100644
--- a/clang/test/CodeGenCXX/microsoft-abi-vbtables.cpp
+++ b/clang/test/CodeGenCXX/microsoft-abi-vbtables.cpp
@@ -537,5 +537,5 @@
 
 extern template class B<int>;
 template B<int>::B();
-// CHECK-DAG: @"\01??_8?$B@H@Test30@@7B@" = external unnamed_addr constant [2 x i32]{{$}}
+// CHECK-DAG: @"\01??_8?$B@H@Test30@@7B@" = linkonce_odr unnamed_addr constant [2 x i32] [i32 0, i32 4], comdat
 }