[OMPT] Adapt OMPT callbacks for tasks to handle untied tasks correctly

The ompt/tasks/task_types.c testcase did not test untied tasks properly. Now,
frame addresses are tested and two scheduling points are added at which the
task can switch to another thread. Due to scheduling effects, the frame address
could be NULL.

This needed a restructure of the way OMPT callbacks are called.
__ompt_task_finish() now as an extra parameter, whether a task is completed.
Its invocation has been moved into __kmp_task_finish(). Thus, the order of the
writes to the frame addresses is not subject to scheduling effects anymore.

Patch by Simon Convent

Reviewed by: protze.joachim, hbae

Subscribers: openmp-commits

Differential Revision: https://reviews.llvm.org/D49181

llvm-svn: 338145