[OMPT] Add ompt_event_task_switch event into OMPT/OpenMP

The patch adds support for ompt_event_task_switch into LLVM/OpenMP. Note that
the patch has also updated the signature of ompt_event_task_switch to
ompt_task_pair_callback_t (rather than the previous ompt_task_switch_callback_t).

Patch by Harald Servat

Differential Revision: http://reviews.llvm.org/D14566

llvm-svn: 252761