[OpenMP][OMPT] Fix ompt_get_task_memory implementation

Since td_allow_completion_event is a member of the taskdata struct, not all
firstprivate/shared variables are stored at the end of the task memory
allocation. Simply report the whole allocation instead.

Furthermore, the function should always return 0 since in no case there is
another block to report.

Differential Review: https://reviews.llvm.org/D158080