[llvm] [OpenMP] Update atomic helpers to just use headers (PR #122185)

[Joseph Huber via llvm-commits]

jhuber6 wrote:

> See comment, if this doesn't break anything, sure. What about
> 
> ```
> /// Atomically increment \p *Addr and wrap at \p V with \p Ordering semantics.
> uint32_t inc(uint32_t *Addr, uint32_t V, OrderingTy Ordering,
>              MemScopeTy MemScope = MemScopeTy::all);
> ```

Figured I'd just leave that one where it is since it doesn't have any template properties and it's weird.

https://github.com/llvm/llvm-project/pull/122185