[clang-tools-extra] [clang-tidy] support expect no diagnosis test (PR #91293)

[Julian Schmidt via cfe-commits]

https://github.com/5chmidti approved this pull request.

Overall LGTM, just two small nits. 

W.r.t flag or no flag: This seems to be so rarely used (judging from the single test file that needed modification), that not having the flag is fine IMO. A check writer doesn't have to know about this small test infra detail, and it's also not *that* surprising, that if there are no `CHECK` messages, none are expected. Similar to the `implicit check-not`.

However, I think I'd be best to get @PiotrZSL's thoughts as well

https://github.com/llvm/llvm-project/pull/91293