[OpenMP] Add omp_pause_resource* API

Add omp_pause_resource and omp_pause_resource_all API and enum, plus stub for
internal implementation. Implemented callable helper function to do local pause,
and added basic functionality for hard and soft pause.

Patch by Terry Wilmarth

Differential Revision: https://reviews.llvm.org/D55078

llvm-svn: 351372