[libcxx-commits] [libcxx] [libc++][modules] Add using_if_exists attribute (#77559) (PR #78909)

[Mark de Wever via libcxx-commits]

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

Thanks a lot for testing and supplying this patch! I expect the CI passes once it does I'll land the patch.

https://github.com/llvm/llvm-project/pull/78909