[Distributed] Implement witnesses for sync or non-throw dist reqs
[Distributed] generic and inner test; without one edge case
[Distributed] fix distributed_thunk test; unsure about those extra hops, could remove later
[Distributed] Remove type pretending in getSILFunctionType; it is not needed
It seems our constant replacement in the earlier phases is enough, and
we don't need this trick at all.
[Distributed] Use thunk when calling cross-actor on DA protocols
