[OpenMP] Clean up load balancing dynamic mode
This patch cleans up the bookkeeping code for the load balancing dynamic mode.
When a thread is moved to or from the thread pool, the th_active_in_pool flag
and the kmp_thread_pool_active_nth global counter are both updated. This
removes the need for the corrective code in the main wait loop. Another global
counter, kmp_thread_pool_nth, was removed completely, as it was only used for
debugging, but was not under KMP_DEBUG.
Patch by Terry Wilmarth
Differential Revision: https://reviews.llvm.org/D59508