[clang] [clang][NFC] Mark CWG2629 as implemented and add a test (PR #195490)

[Victor Chernyakin via cfe-commits]

localspook wrote:

@Endilll Would you prefer that I did multiple DRs per PR (assuming none of them require compiler changes)? I've been doing 1 per PR so far, and I worry I'm being annoying by opening a bunch of tiny PRs.

https://github.com/llvm/llvm-project/pull/195490