[libclc] [NFC][libclc] Merge atomic extension built-ins with identical name into a single file (PR #134489)

[Fraser Cormack via cfe-commits]

https://github.com/frasercrmck commented:

Thanks. It seems like a guard mistake in a header has crept into the new headers. Other than that, LGTM.

https://github.com/llvm/llvm-project/pull/134489