First, and I really should've checked this, but global actors do not
require `shared` to return `Self`; adjust the logic to propagate the
right formal type to the erasure logic.
Second, handle attempts to erase the isolation of something isolated to
a distributed actor using the magic Actor conformance that Doug added.
This only works when the actor is local, but it shouldn't be difficult to
enforce that we only attempt to erase isolation what that's true --- we
need to prevent partial application of distributed actors, and we need to
disallow explicit isolated captures of distributed actors when we're not
currently isolated to it. Otherwise, it's not possible to get an
@isolated(any) function value with an isolation that isn't the current
function's isolation, which means it always has to be local.
Fixed rdar://123734019
