[clang] [Attributes] Support Attributes being declared as supporting an experimental late parsing mode "extension" (PR #88596)

[Erich Keane via cfe-commits]

https://github.com/erichkeane approved this pull request.

This looks fine enough to me. Aaron is away this week, so you can either let him review this when he gets back, or just merge this and let him make comments on followups/on the commit when he gets back.

https://github.com/llvm/llvm-project/pull/88596