[clang] [llvm] CodeGen: Replace TargetOptions::ThreadModel with module flag (PR #223157)

[Yonah Goldberg via cfe-commits]

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

Agree we should probably delete `LowerAtomicPass` if we're adding this machinery in `AtomicExpandPass`.

https://github.com/llvm/llvm-project/pull/223157