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

[Dan Liew via cfe-commits]

delcypher wrote:

@AaronBallman Thanks for the feedback

> 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`?

I had a think about it and I'll add a release note but note that it doesn't do anything and then update the note in the PR that introduces the first use of `LateAttrParseExperimentalExt`. I think the commit history will be cleanest this way.

> 
> 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?

So we have this work-in-progress PR https://github.com/llvm/llvm-project/pull/87596 up that will use `LateAttrParseExperimentalExt`. Although we could have landed these two PRs as one I thought it would be better to split them up to make reviewing more manageable.




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