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

[Erich Keane via cfe-commits]

erichkeane wrote:

Did a message get deleted here?  I saw in my email that you pointed out there is a memory leak because of this patch.  (https://lab.llvm.org/buildbot/#/builders/239/builds/7043)

If so, we probably should revert this until that gets figured out.

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