[flang][openmp] Required clauses are allowed

Summary:
This patch fix a problem where clause needed to be in the allowed set even
they were in the required set. A required clause is allowed obvisouly. This allow
to remove the duplicate in OMP.td

Reviewers: kiranchandramohan, DavidTruby, richard.barton.arm, jdoerfert, sscalpone, kiranktp, ichoyjx

Reviewed By: kiranchandramohan

Subscribers: yaxunl, guansong, sstefan1, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D84353