[openmp] Remove OMPConstants.cpp and replace it by OMP.cpp generated by tablegen

Summary:
Diff D83176 moved the last piece of code from OMPConstants.cpp and now this file was only
useful to include the tablegen generated file. This patch replace OMPConstants.cpp with OMP.cpp
generated by tablegen.

Reviewers: sstefan1, jdoerfert, jdenny

Reviewed By: sstefan1

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

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D83583