[libcxx-commits] [libcxx] [libc++][NFC] Simplify features for detecting atomics' support. (PR #75553)

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

I think this change looks good, modulo other comments.

Unrelated to this change... but... Are we really using the behavior of our atomic implementation to determine the expected behavior of our atomic implementation? That's terrifying and terrible. We shouldn't do that. Yucky.

https://github.com/llvm/llvm-project/pull/75553