[clang] [Clang] Re-write codegen for atomic_test_and_set and atomic_clear (PR #121943)

[Jordan Rupprecht via cfe-commits]

rupprecht wrote:

Thanks! The issues I mentioned on #120449 seem to be covered by tests, so LGTM. I will defer to others for a more qualified review though.

https://github.com/llvm/llvm-project/pull/121943