swift234

When type-checking downcasts, just optimistically
assume that we know about all the optional types in play.

This isn't correct for types that can dynamically be
more optional than they appear statically, e.g. archetypes
and existentials, but it's the easiest thing to do,
and there are workarounds.  I filed rdar://16374053
to handle doing the right thing.

Swift SVN r15254