[clang] Turn 'counted_by' into a type attribute and parse it into 'CountAttributedType' (PR #78000)

[Aaron Ballman via cfe-commits]

https://github.com/AaronBallman approved this pull request.

Thank you for this, it's exciting to see more bounds safety being added to Clang!

I think everything looks good to me (we can clean up any surprises in follow-ups at this point). I'm presuming you'll add a release note & docs once more of the functionality is place?

https://github.com/llvm/llvm-project/pull/78000