[OpenMPOPT][NFC] Introducing OMPInformationCache.

Summary:
Introduction of OpenMP-specific information cache based on Attributor's `InformationCache`. This should make it easier to share information between them.

Reviewers: jdoerfert, JonChesterfield, hamax97, jhuber6, uenoku

Subscribers: yaxunl, hiraditya, guansong, uenoku, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D81798