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

[Erich Keane via cfe-commits]

erichkeane wrote:

> > ...that the template cases are going to be REALLY noisy
> 
> @erichkeane It looks like that's why I have two failed tests from `llvm-libc++-shared-cfg-in` due to the use of qualifiers

Yep, that looks to be exactly the case!  I'd be interested to see if @ldionne would be willing to accept a `std::remove_cv_t` or something on those.  This diagnostic is likely going to require/cause a level of const/volatile cleanliness in base classes that no one has ever really thought about before.

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