[OpenMP] Introduce new flags to assert thread and team usage in the runtime

This patch adds two flags to be supported for the new runtime. The flags
are `-fopenmp-assume-threads-oversubscription` and
-fopenmp-assume-teams-oversubscription`. These add global values that
can be checked by the work sharing runtime functions to make better
judgements about how to distribute work between the threads.

Reviewed By: jdoerfert

Differential Revision: https://reviews.llvm.org/D111348