[openmp] Move Directive and Clause helper function to tablegen

Summary:
Follow up to D81736. Move getOpenMPDirectiveKind, getOpenMPClauseKind, getOpenMPDirectiveName and
getOpenMPClauseName to the new tablegen code generation. The code is generated in a new file named OMP.cpp.inc

Reviewers: jdoerfert, jdenny, thakis

Reviewed By: jdoerfert, jdenny

Subscribers: mgorny, yaxunl, hiraditya, guansong, sstefan1, llvm-commits, thakis

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D82405