swift234

Instead of assuming that the return type of a closure
with no returns *must* be (), add a defaulting constraint
so that it will be inferred as () in the absence of
other possibilities.

The chief benefit here is that it allows better QoI when
the user simply hasn't yet written the return statement.

Doing this does regress a corner case where an attempt
to recover from an uncalled function leads to the
type-checker inferring a result for a closure that
doesn't make any sense at all.

Swift SVN r30476