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

[Dan Liew via cfe-commits]

delcypher wrote:

@nathanchance Thanks for reporting this. I'm going to have a quick go at reproducing this to see if fixing this is straight forward. If it is not I will revert this PR and then we can re-land this change with the problem you reported fixed.

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