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

[Erich Keane via cfe-commits]

erichkeane wrote:

> @erichkeane I posted the comment on the wrong PR. The memory leak is in #90786 not this PR so I deleted the comment i made here. #90786 has already been reverted.

Thanks!  I am still catching up on emails, and I think I figured that out literally moments ago :) 

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