[OpenMP] libomp: implement nteams-var and teams-thread-limit-var ICVs

The change includes OMP_NUM_TEAMS, OMP_TEAMS_THREAD_LIMIT env variables,
omp_set_num_teams, omp_get_max_teams, omp_set_teams_thread_limit,
omp_get_teams_thread_limit routines.

Differential Revision: https://reviews.llvm.org/D95003