[OMPT] Fix OMPT callbacks for the taskloop construct and add testcase

Fix the order of callbacks related to the taskloop construct.
Add the iteration_count to work callbacks (according to the spec).
Use kmpc_omp_task() instead of kmp_omp_task() to include OMPT callbacks.
Add a testcase.

Patch by Simon Convent

Reviewed by: protze.joachim, hbae

Subscribers: openmp-commits

Differential Revision: https://reviews.llvm.org/D47709

llvm-svn: 338146