[clang] [Clang] refine diagnostic for redundant qualifiers on base class type to handle singular and plural cases (PR #125943)

[Erich Keane via cfe-commits]

https://github.com/erichkeane approved this pull request.

Please improve the commit message to better explain what is going on, else LGTM.

https://github.com/llvm/llvm-project/pull/125943