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

[Yeoul Na via cfe-commits]

rapidsna wrote:

@AaronBallman Thank you! The -fbounds-safety documentation is currently shown https://clang.llvm.org/docs/BoundsSafety.html, but yes, we're planning to add the feature in the release note once we have more functionalities in place.

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