[libc++] Don't implement <stdatomic.h> before C++23 (#123130) https://github.com/llvm/llvm-project/pull/95498 implemented a libc++ extension where <stdatomic.h> would forward to <atomic> even before C++23. Unfortunately, this was found to be a breaking change (with fairly widespread impact) since that changes whether _Atomic(T) is a C style atomic or std::atomic<T>. In principle, this can even be an ABI break. We generally don't implement extensions in libc++ because they cause so many problems, and that extension had been accepted because it was deemed pretty small and only a quality of life improvement. Since it has widespread impact on valid C++20 (and before) code, this patch removes the extension before we ship it in any public release.
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