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

[Dan Liew via cfe-commits]

delcypher wrote:

@erichkeane Thanks for approving. I'll rebase, check this builds and then land this.

@AaronBallman When you're back please let me know if there's any follow up changes you want.

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