[libcxx-commits] [libcxx] [libc++] Fix diagnostic for <stdatomic.h> before C++23 (PR #83351)

[Louis Dionne via libcxx-commits]

ldionne wrote:

I'm not sure how that ever worked since we did have an `#error` before, it just didn't provide a good diagnostic?

https://github.com/llvm/llvm-project/pull/83351