[OMPT] Fix wrong parent_task_id in serialized parallel_begin with GCC

Without this patch a simple '#pragma omp parallel num_threads(1)' leads to
ompt_event_parallel_begin: parent_task_id=3, [...], parallel_id=2, [...]
ompt_event_parallel_end: parallel_id=2, task_id=4, [...]

Differential Revision: http://reviews.llvm.org/D16714

llvm-svn: 263940