[OMPT]: Fix the order of implicit_task_end_events

For implcit barriers in simple parallel for loops, the order of the OMPT events
was wrong. The barrier_{begin,end} events came after the implcit_task_end
event for the implcit barrier at the end of the parallel region. This is wrong
because the implicit task executes the barrier before ending. This patch fixes
the order of the event: It will be triggerd now just before
__kmp_pop_current_task_from_thread() is called.

Patch by Tim Cramer

Differential Revision: http://reviews.llvm.org/D16347