[clang] [llvm] [IR] Remove volatile from nosync (PR #194391)

[via cfe-commits]

gonzalobg wrote:

Discussed with @nikic offline. I had missed that `nosync` does not have any impact on `atomic` `monotonic` and for `volatile` still allows targets to use `volatile` to create communication across threads. What `nosync` would prevent  is using `volatile` to introduce other memory ordering like `acq_rel` due to implementing some barrier (@jyknight example). Such SW would need to insert compiler fences around volatile (e.g. signal fences).

I think I am fine with this change.

https://github.com/llvm/llvm-project/pull/194391