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

[Ilya Biryukov via cfe-commits]

ilya-biryukov wrote:

> @ilya-biryukov You suggested previously (https://github.com/llvm/llvm-project/pull/118567/files#r1868729303) to introduce a separate enum, but I didn't manage to do it so far. Instead, I added new strings to `AttributeDeclKind`. My thinking has been:

I got confused and thought the `AttributeDeclKind` is something that's used outside of diagnostics and diagnostics simply reuse it. But that's not the case, so the change LG!

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