[clang] [Attributes] Support Attributes being declared as only supporting late parsing when passing an experimental feature flag (PR #88596)

[via cfe-commits]

https://github.com/Sirraide requested changes to this pull request.

Not too familiar w/ the overall context of this, so I can only comment on the implementation.

This should also add some tests that actually use experimentally late-parsed attributes w/ the flag explicilty enabled/disabled.

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