<atomic>: Remove workarounds for LLVM-46595

[CaseyCarter]

LLVM-46595 requests support in Clang to mirror MSVC's addition of _InterlockedMeow64 intrinsics in x86 mode. The bug has been fixed on trunk, so we can remove our workarounds in <atomic> when we update our required Clang version (presumably) to LLVM 11.

[StephanTLavavej]

Wow, llvm/llvm-project@82206e7 was implemented incredibly quickly - thank you @topperc! 😸

[StephanTLavavej]

The MSVC compiler had to revert its addition of the intrinsics due to compatibility with the Win8.1 SDK, so we're simply going to avoid them for all compilers. 😿