[OpenMP] Fix race condition in the completion/freeing of detached tasks

Spurious assertion failures are symptoms of a race condition for the handling
of detached tasks:
Assertion failure at kmp_tasking.cpp(3744): taskdata->td_flags.complete == 1.
Assertion failure at kmp_tasking.cpp(710): taskdata->td_flags.executing == 0.

in the case of detach=true, all accesses to taskdata in __kmp_task_finish need
to happen before (~line 873):

taskdata->td_flags.proxy = TASK_PROXY;

This assignment signals to __kmp_fulfill_event, that the task will need to be
freed there. So, conceptionally the ownership of taskdata is moved.

Reviewed By: AndreyChurbanov

Differential Revision: https://reviews.llvm.org/D79702