[openmp] Use Directive_enumSize instead of OMPD_unknown position

Summary:
Previously OMPD_unknown was last item in the Directive enumeration and its position was
used in various comparison and assertion. With the new Directive enumeration, this should be
change with  llvm::omp::Directive_enumSize. This patch fix two place where it was not done in
D81736.

Reviewers: vdmitrie, jdoerfert, jdenny

Reviewed By: jdoerfert

Subscribers: yaxunl, guansong, sstefan1, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D82518