[clang] [clang][CodeGen] Emit atomic IR in place of optimized libcalls. (PR #73176)

[Eli Friedman via cfe-commits]

efriedma-quic wrote:

When I said "inconsistencies", I just meant the way the responsibility for lowering atomics is split between LLVM and clang; I didn't mean anything was actually broken.

https://github.com/llvm/llvm-project/pull/73176