[openmp][openacc] Check for duplicate clauses for directive

Check for duplicate clauses associated with directive. Clauses can appear only once
in the 4 lists associated with each directive (allowedClauses, allowedOnceClauses,
allowedExclusiveClauses, requiredClauses). Duplicates were already present (removed with this
patch) or were introduce in new patches by mistake (D89861).

Reviewed By: kiranchandramohan

Differential Revision: https://reviews.llvm.org/D90241