[OpenMP] Fix for smaller team sizes inside teams construct.

When a team nested inside a teams construct is allocated, it is
allocated to a size specified by the teams thread_limit.  In the case
where any mechanism that might not grant the full thread_limit is in
use, we may get a smaller team.  This possibility was not reflected in
the code when using the th_teams_size.nth value stored on the master
thread for the team. This value was never updated even when t_nproc on
the team itself was different.  I added a line to update it shortly
before the team is forked.

Added a simple teams test that uses KMP_DYNAMIC_MODE=random to mimic
allocating teams with sizes <= thread_limit. Eventually, this
will segfault without the fix in this commit.

Differential Revision: https://reviews.llvm.org/D139960