swift234

Implement explicit (unchecked!) downcasting for class types.

This introduces support for the syntax

  Derived(baseObj)

to downcast from a class type to one of its subclasses. This still
needs more language design and implementation work, including:
  - This overloads the X(y) syntax again, which already means either
  "coerce y to type X, performing implicit conversions if necessary"
  or "construct a value of type X from y". It's no actually ambiguous,
  because the first case won't apply for downcasts and the second case
  is limited to value types, but it makes me wonder whether we want a
  different syntax for the first case.

  - We need this to be a checked cast, but don't have the runtime
    infrastructure to do so yet. I've left this as a FIXME.

However, the Objective-C importer is fairly useless because everything
that creates an object returns an "id", "id" maps to "NSObject", and
then the type system doesn't let you get from NSObject back to the
type you care about. So, this lets you explicitly do the cast.



Swift SVN r3279