Add initial support for OpenMP 4.5 task priority feature

The maximum task priority value is read from envirable: OMP_MAX_TASK_PRIORITY.
But as of now, nothing is done with it.  We just handle the environment variable
and add the new api: omp_get_max_task_priority() which returns that value or
zero if it is not set.

Differential Revision: http://reviews.llvm.org/D17411

llvm-svn: 261908