[clang] [llvm] [LLVMABI] Handle Atomic types in the LLVM ABI library (PR #221367)

[Erich Keane via cfe-commits]

https://github.com/erichkeane approved this pull request.

FE parts look good, I don't know much of the ABI library, but the code looks consistent with the rest of the stuff around it/the logic seems reasonable.  That said, it would be awesome if someone more familiar with it took a look.

https://github.com/llvm/llvm-project/pull/221367