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

[Rainer Orth via cfe-commits]

rorth wrote:

I'm pretty certain this patch broke the [Solaris/sparcv9 buildbot](https://lab.llvm.org/buildbot/#/builders/72/builds/2541).

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