[clang] [Clang] emit -Wignored-qualifiers diagnostic for cv-qualified base classes (PR #121419)

[Erich Keane via cfe-commits]

erichkeane wrote:

> @erichkeane @ldionne should the `libcxx` tests ([common_reference.compile.pass.cpp](https://github.com/llvm/llvm-project/blob/main/libcxx/test/std/utilities/meta/meta.trans/meta.trans.other/common_reference.compile.pass.cpp), [apply_extended_types.pass.cpp](https://github.com/llvm/llvm-project/blob/main/libcxx/test/std/utilities/tuple/tuple.tuple/tuple.apply/apply_extended_types.pass.cpp)) be updated or should the new warnings/errors be treated as the new baseline?

This is entirely a @ldionne question, though I see in github that he's on vacation at the moment, so we'll have to wait until he returns.

https://github.com/llvm/llvm-project/pull/121419