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

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MagentaTreehouse wrote:

I suggest backporting to the 20 release. Wdyt? (If we decide so, the release notes would go into 20 instead of main)

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