[clang] [lldb] [Clang] Improve Sema diagnostic performance for __builtin_counted_by_ref (PR #116719)

[Aaron Ballman via cfe-commits]

https://github.com/AaronBallman approved this pull request.

LGTM modulo the request for a helper function (that two reviewers asked for the same thing is a pretty reasonable indication that we should probably have the helper, but if you feel strongly about it, I won't block the PR over it).

https://github.com/llvm/llvm-project/pull/116719