[clang-tools-extra] [clang-tidy] Add new check bugprone-tagged-union-member-count (PR #89925)

[Kristóf Umann via cfe-commits]

Szelethus wrote:

My say doesn't quite have the same weight here are many others, but I see no problems with this patch after so many rounds of reviews -- would it be okay to merge?

https://github.com/llvm/llvm-project/pull/89925