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

[Aaron Ballman via cfe-commits]

https://github.com/AaronBallman commented:

We should probably add a release note to tell users about the new command line option, unless you think it makes more sense to do so after something uses `LateAttrParseExperimentalExt`?

One concern I have is that nothing is testing the new parsing behavior in ParseDecl.cpp, but I presume that will get tested shortly by making use of the new form?

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