When a target task finishes and it tries to access the th_task_team from the
threads in the team where it was created, th_task_team can be NULL or point to
a different place when that thread started a nested region that is still
running. Finding the exact task_team that the threads were using is difficult
as it would require to unwind the task_state_memo_stack. So a new field was added
in the taskdata structure to point to the active task_team when the task was
created.

llvm-svn: 260615