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

[Dan Liew via cfe-commits]

delcypher 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.

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