[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:

Just some general comments, most of which apply to multiple files.

In principle I like the change, though - thank you.

I also think the idea of making overriding based solely on filename is interesting.

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