Re-instate -Wweak-template-vtables as a no-op flag

Follow-up to 8c13680524 to allow a less
abrupt migration for users.

Differential revision: https://reviews.llvm.org/D112704

clang/docs/ReleaseNotes.rst

@@ -67,7 +67,8 @@ New Compiler Flags
 Deprecated Compiler Flags
 -------------------------
 
-- ...
+- -Wweak-template-vtables has been deprecated and no longer has any effect. The
+  flag will be removed in the next release.
 
 Modified Compiler Flags
 -----------------------

clang/include/clang/Basic/DiagnosticSemaKinds.td

@@ -1647,6 +1647,9 @@ def warn_weak_vtable : Warning<
   "%0 has no out-of-line virtual method definitions; its vtable will be "
   "emitted in every translation unit">,
   InGroup<DiagGroup<"weak-vtables">>, DefaultIgnore;
+def warn_weak_template_vtable : Warning<
+  "this warning is no longer in use and will be removed in the next release">,
+  InGroup<DiagGroup<"weak-template-vtables">>, DefaultIgnore;
 
 def ext_using_undefined_std : ExtWarn<
   "using directive refers to implicitly-defined namespace 'std'">;

clang/test/SemaCXX/warn-weak-vtables.cpp

@@ -3,6 +3,9 @@
 // Check that this warning is disabled on MS ABI targets which don't have key
 // functions.
 // RUN: %clang_cc1 %s -fsyntax-only -triple %ms_abi_triple -Werror -Wweak-vtables
+//
+// -Wweak-template-vtables is deprecated but we still parse it.
+// RUN: %clang_cc1 %s -fsyntax-only -Werror -Wweak-template-vtables
 
 struct A { // expected-warning {{'A' has no out-of-line virtual method definitions; its vtable will be emitted in every translation unit}}
   virtual void f() { }