[openmp] Use switch in isAllowedClauseForDirective instead of multiple if

Summary:
Change the test in isAllowedClauseForDirective from if with multiple conditions
to a main switch on directive and then switches on clause for each directive. Version
check is still done with a condition in the return statment.

Reviewers: jdoerfert, jdenny

Reviewed By: jdenny

Subscribers: yaxunl, guansong, sstefan1, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D83363