[OpenMP][OMPT] Fix and add event callbacks for detached tasks

The OpenMP spec has the task-fulfill event for a call to omp_fulfill_event.
If the task did not yet finish execution, ompt_task_early_fulfill is used,
otherwise ompt_task_late_fulfill.
If a task does not complete, when the execution finishes (i.e., the task goes
in detached mode), ompt_task_detach instead of ompt_task_complete must be
used, when the next task is scheduled.

A test for both cases is included, which only work with clang-11+

Reviewed By: hbae

Differential revision: https://reviews.llvm.org/D80843