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

[Piotr Zegar via cfe-commits]

https://github.com/PiotrZSL requested changes to this pull request.

This check may not have to many users, but that's fine.
I'm 100% sure that I seen such structures already.

As for code changes, please fix pointed out nits and after that checkout probably could be merged.

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