[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

llvm-svn: 357927