[clang] [BoundsSafety][NFC] Add counted_by type-shape validation helper (PR #227774)

[Dan Liew via cfe-commits]

https://github.com/delcypher requested changes to this pull request.

Thanks for addressing my first set of comments. I noticed a few more minor issues. Once we've resolved those we should take this PR out of being a draft and add more reviewers.

https://github.com/llvm/llvm-project/pull/227774