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

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MagentaTreehouse wrote:

> Happy to adjust this @MagentaTreehouse for LLVM 20 - what would that require?

@lyledean1 I think the process is that the release notes are left unchanged in this PR; after it is merged we add this to [LLVM 20.X Release](https://github.com/llvm/llvm-project/milestone/26) milestone, open a backport PR by cherry-picking the commit ([Example](https://github.com/llvm/llvm-project/pull/131450)), and add changes to the release notes there. 

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