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

[Maksim Ivanov via cfe-commits]

emaxx-google 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:

1. If we create a enum, I'm not clear on how it'd be different from `AttributeDeclKind`.
  * This `AttributeDeclKind` is already used both by Parser and Sema - same is for our callsites in question.
  * Not sure about how to name the items differently - they do seem very close.
2. Another idea was to include the old enum's items into the new one.
  * But I don't know how, technically, could we do it without copy-pasting the enum items and `%select` arguments.

But I'm open to any suggestions; thanks!

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