[OpenMP][libomptarget] Use shared memory variable for tracking parallel level

Summary: Replace existing infrastructure for tracking parallel level using global memory with a per-team shared memory variable. This minimizes the impact of the overhead of tracking the parallel level for non-nested cases.

Reviewers: ABataev, caomhin

Reviewed By: ABataev

Subscribers: guansong, openmp-commits

Differential Revision: https://reviews.llvm.org/D55773

llvm-svn: 350747