[clang] [Clang] add emit -Wignored-cv-qualifiers diagnostic for cv-qualified base classes (PR #132116)

[Erich Keane via cfe-commits]

erichkeane wrote:

> > The more I think about it, the more I think that we should modify the warning to only happen on EXPLICIT qualifiers. So:
> > ```
> > struct Base{};
> > using CBase = const Base;
> > 
> > struct D1 : const Base {}; // DOES warn
> > struct D2 : CBase{}; // a DIFFERENT warning (same wording perhaps?) under a different group.
> > template<typename T> Templ: const T{};
> > Templ<Base> t; // Causes same warning as D1.
> > template<typename T> Templ2 : T{};  
> > Templ2<CBase> t; // Causes same warning as D2.
> > ```
> 
> Adding qualifiers directly in base specifiers is invalid (and all major compilers correctly reject this). See grammar in [[class.derived.general]/1](https://timsong-cpp.github.io/cppwp/n4950/class.derived#general-1). Qualifiers can indirectly come from `decltype`, template parameters, type aliases and so on.

Ah, cool.  I didn't realize we rejected that already. Then I think adding this to a new flag is sufficient, so probably OK after Aaron + My comments are addressed.

https://github.com/llvm/llvm-project/pull/132116