Avoid deadlock with COI

When an asynchronous offload task is completed, COI calls the runtime to queue
a "destructor task".  When the task deques are full, a dead-lock situation
arises where the OpenMP threads are inside but cannot progress because the COI
thread is stuck inside the runtime trying to find a slot in a deque.

This patch implements the solution where the task deques doubled in size when
a task is being queued from a COI thread.

Differential Revision: http://reviews.llvm.org/D20733

llvm-svn: 271319