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

[Johannes Doerfert via llvm-commits]

https://github.com/jdoerfert commented:

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);
```

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