Fixes and improvements to tasking in barriers

1) When the number of threads in a team increases, new threads need to have all
   their barrier struct fields initialized. We were missing the parent_bar and
   team fields.
2) For non-forkjoin barriers, we now do the __kmp_task_team_setup before the
   gather. The setup now sets up the task_team that all the threads will switch
   to after the barrier, but it needs to be done before other threads do the
   switch.
3) Remove an unneeded assignment of tt_found_tasks in task team free function.

Differential Revision: http://reviews.llvm.org/D14456

llvm-svn: 252486