[OPENMP50]Allow nonmonotonic modifier for all schedule kinds.

Summary:
According to OpenMP 5.0, nonmonotonic modifier can be used with all
schedule kinds, not only dynamic and guided as in OpenMP 4.5.

Reviewers: jdoerfert

Subscribers: yaxunl, guansong, sstefan1, cfe-commits, caomhin

Tags: #clang

Differential Revision: https://reviews.llvm.org/D82026