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

[Aaron Ballman via cfe-commits]

https://github.com/AaronBallman commented:

Thank you for your patience! Overall, I don't spot any major concerns, but I did spot some minor nits. I've added @erichkeane to the review as attributes code owner for a second set of eyes on those changes specifically.

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