[clang] [clang] Refactor attr diagnostics to use %select (PR #122473)

[Ilya Biryukov via cfe-commits]

https://github.com/ilya-biryukov commented:

This looks like a clear improvement. Just a quick check on whether we can remove the `_str` diagnostics completely (see the relevant comment)

https://github.com/llvm/llvm-project/pull/122473