[OpenMP] libomp: implemented task priorities.

Before this patch task priorities were ignored, that was a valid implementation
as the task priority is a hint according to OpenMP specification.

Implemented shared list of sorted (high -> low) task deques one per task
priority value. Tasks execution changed to first check if priority tasks ready
for execution exist, and these tasks executed before others;
otherwise usual tasks execution mechanics work.

Differential Revision: https://reviews.llvm.org/D119676