dispatch_source_create is marked up as non-null but can actually return
null in programming error issues. In order to maintain the nullabilty
contract, these issues will now cause the overlay to assert and crash
rather than return a null pointer out of a non-null call.
Resolves: <rdar://problem/39937177>
We still have a bunch of redeclarations of Dispatch functions to avoid
the automatic bridging of dispatch_data_t and dispatch_block_t, but
mostly this is a vast reduction in complexity (and increase in safety).
