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

[hans]

Summary

Follow-up to https://github.com/llvm/llvm-project/commit/8c136805242014b6ad9ff1afcac9d7f4a18bec3f to allow a less abrupt migration for users.

[thakis]

Thanks!

clang/docs/ReleaseNotes.rst

 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
 -----------------------
 
 - Support has been added for the following processors (``-mcpu`` identifiers in parentheses):
 
   - RISC-V SiFive E20 (``sifive-e20``).
   - RISC-V SiFive E21 (``sifive-e21``).

clang/include/clang/Basic/DiagnosticSemaKinds.td

def err_consteval_override : Error<
   "consteval function %0 cannot override a non-consteval function">;
 def err_non_consteval_override : Error<
   "non-consteval function %0 cannot override a consteval function">;
 
 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'">;
 
 // C++ exception specifications
 def err_exception_spec_in_typedef : Error<
   "exception specifications are not allowed in %select{typedefs|type aliases}0">;
 def err_distant_exception_spec : Error<

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

 // RUN: %clang_cc1 %s -fsyntax-only -verify -triple %itanium_abi_triple -Wweak-vtables
 //
 // 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() { } 
 };
 
 template<typename T> struct B {
   virtual void f() { } 
 };