[llvm] [llvm][OpenMP][NFC] Cleanup AtomicInfo (PR #119199)

[Krzysztof Parzyszek via llvm-commits]

kparzysz wrote:

Please fix the comments at the top of both files.  Both say "Atomic.h", and the first line is broken into two.

I think that too many includes are now removed.  For example, the header file uses `IntegerType::get`, but there is no `#include "llvm/IR/DerivedTypes.h"`, or `#include <utility>` for `std::pair`.  I believe the preference is to IWYU, but I'll defer to @nikic to decide.

https://github.com/llvm/llvm-project/pull/119199