dd new OpenMP 4.5 schedule clause modifiers (monotonic/non-monotonic) feature

The monotonic/non-monotonic flags are sent to the runtime via the sched_type by
setting the 30th (non-monotonic) or 29th (monotonic) bit in the sched_type.
Macros are added to probe if monotonic or non-monotonic is specified
(SCHEDULE_HAS_[NON]MONOTONIC & SCHEDULE_HAS_NO_MODIFIERS)
and also to to get the base sched_type (SCHEDULE_WITHOUT_MODIFIERS)

Currently, nothing is done with the modifiers.

Also, this patch adds some comments on the use of the enumerations in at least
 one place where it is subtle.

Differential Revision: http://reviews.llvm.org/D17406

llvm-svn: 261906