[libcxx-commits] [libcxx] [libc++] Add tests for LWG3949 and mark it as done (PR #208272)

[Nikolas Klauser via libcxx-commits]

https://github.com/philnik777 approved this pull request.

I'm somewhat on the fence whether this is complete or nothing to do, but given that it seems it was required before and accidentally deleted nothing to do seems fine.

https://github.com/llvm/llvm-project/pull/208272