[coroutines] Handle get_return_object_on_allocation_failure

Summary:
If promise_type has get_return_object_on_allocation_failure defined,
check if an allocation function returns nullptr, and if so,
return the result of get_return_object_on_allocation_failure().

Reviewers: rsmith, EricWF

Reviewed By: EricWF

Subscribers: mehdi_amini, cfe-commits

Differential Revision: https://reviews.llvm.org/D31399

llvm-svn: 298891