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

[Yeoul Na via cfe-commits]

rapidsna wrote:

@AaronBallman @erichkeane gentle reminder. Does my reasoning [above](https://github.com/llvm/llvm-project/pull/78000#issuecomment-1932496432) to make a new sugar type make sense?

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