[OpenMP] Add GOMP 5.0 loop entry points

This patch adds missing GOMP_5.0 loop entry points which incorporate
new non-monotonic default into entry point name.  Since monotonic
schedules are a subset of nonmonotonic, it is acceptable to use
monotonic as the implementation.  This patch simply has the nonmonotonic
(and possibly non-monontonic) versions of the loop entry points as
wrappers around the monotonic ones.

Differential Revision: https://reviews.llvm.org/D73922