[clang] [CIR] Atomic fetch operation (PR #161631)

[Sirui Mu via cfe-commits]

Lancern wrote:

> @Lancern Now that https://github.com/llvm/llvm-project/pull/160330 has been merged, is this ready to be rebased?

@andykaylor Thanks for pinging. I'll update this PR soon.

https://github.com/llvm/llvm-project/pull/161631