[LAA] Move CheckingPtrGroup/PointerCheck outside class (NFC).

This allows forward declarations of PointerCheck, which in turn reduce
the number of times LoopAccessAnalysis needs to be included.

Ultimately this helps with moving runtime check generation to
Transforms/Utils/LoopUtils.h, without having to include it there.

Reviewers: anemet, Ayal

Reviewed By: Ayal

Differential Revision: https://reviews.llvm.org/D78458