Pass the frame from the outer yield_once_2 coroutine into the
swift_coro_de/alloc functions and from there into all four de/allocation
witnesses. This enables ABIs which key off of information stored in the
frame.
To enable ABIs which store extra info in the frame, add two new slots to
the coroutine allocator function table. For example, a frame could have
a header containing a context pointer at a negative offset from the
address returned from `swift_coro_alloc_frame`. The frame deallocation
function would then know to deallocate more space correspondingly.
Doing so enables allocators to contain additional context for use by
allocation functions. Because the allocator is already passed to
_swift_coro_alloc, on the fast path (no allocator, popless) no
allocation function is used, and the allocator is passed in the
swiftcoro register, this is cheap.
Allocator structs are passed in to new ABI yield-once coroutines and
contain pointers to functions to de/allocate memory. Here, those
pointers are signed.
When building for back-deployment, emit calls to an open-coded
`_swift_task_dealloc_through` function rather than the runtime
`swift_task_dealloc_through` which doesn't exist on them.
