[OpenMP][NFCI] Introduce llvm/IR/OpenMPConstants.h

Summary:
The new OpenMPConstants.h is a location for all OpenMP related constants
(and helpers) to live.

This patch moves the directives there (the enum OpenMPDirectiveKind) and
rewires Clang to use the new location.

Initially part of D69785.

Reviewers: kiranchandramohan, ABataev, RaviNarayanaswamy, gtbercea, grokos, sdmitriev, JonChesterfield, hfinkel, fghanim

Subscribers: jholewinski, ppenzin, penzn, llvm-commits, cfe-commits, jfb, guansong, bollu, hiraditya, mgorny

Tags: #clang, #llvm

Differential Revision: https://reviews.llvm.org/D69853