[libcxx-commits] [libcxx] [libc++] Enable availability based on the compiler instead of __has_extension (PR #84065)

[Louis Dionne via libcxx-commits]

ldionne wrote:

I will cherry-pick this onto `release/18.x` since this breaks essentially anyone who uses availability markup and is trying to use `-pedantic-errors`.

https://github.com/llvm/llvm-project/pull/84065