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

[Yeoul Na via cfe-commits]

rapidsna wrote:

Gentle reminder @bwendling @AaronBallman. I'd like to land this patch soon, so we can open source more of -fbounds-safety work. Could you please take a look?

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