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

[Dan Liew via cfe-commits]

delcypher wrote:

@rapidsna I've re-implemented this patch with the new semantics that we discussed offline. I have also reworked #87596 to use this new implementation.

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