swift234

Make nil convert to both T? and @unchecked T?.

This was blocked by some type-checker issues:

First, we weren't registering a constraint restriction when
tail-recursing in matchTypes (as opposed to when creating
a disjunction because multiple conversions applied).  Do so,
and move the set of constraint restrictions to the constraint
system in order to make this simpler.  A large amount of similar
solver state is already there, and of course solving the system
already prospectively modifies the constraint graph.

Second, only set up a potential existential conversion when
working with concrete types.  Without this, we would fail to
typecheck conversions to optional protocol types, but not
optional class/struct/whatever types.  It's not clear whether
whether we should ever really be considering conversions when
either of the types is non-concrete.

I believe it was the second fix which removed a need for a !
in the NewArray test case.

Swift SVN r14637