[openmp] Remove OMPConstants.cpp and replace it by OMP.cpp generated by tablegen
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
Differential Revision: https://reviews.llvm.org/D83583