[clang] [Clang] Add __builtin_counted_by_ref builtin (PR #114495)

[Bill Wendling via cfe-commits]

bwendling wrote:

@AaronBallman @rapidsna -- I think I covered all of the comments you made on the other PR (stupid git!). PTAL.

https://github.com/llvm/llvm-project/pull/114495