[flang][openmp] Use common Directive and Clause enum from llvm/Frontend

Summary:
This patch is removing the custom enumeration for OpenMP Directives and Clauses and replace them
with the newly tablegen generated one from llvm/Frontend. This is a first patch and some will follow to share the same
infrastructure where possible. The next patch should use the clauses allowance defined in the tablegen file.

Reviewers: jdoerfert, DavidTruby, sscalpone, kiranchandramohan, ichoyjx

Reviewed By: DavidTruby, ichoyjx

Subscribers: jholewinski, cfe-commits, dblaikie, MaskRay, ymandel, ichoyjx, mgorny, yaxunl, guansong, jfb, sstefan1, aaron.ballman, llvm-commits

Tags: #llvm, #flang, #clang

Differential Revision: https://reviews.llvm.org/D82906