This change enables inheritance constraints such as "T : NSObject",
which specifies that the type parameter T must inherit (directly or
indirectly) from NSObject. One can then implicit convert from T to
NSObject and perform (checked) downcasts from an NSObject to a T. With
this, we can type-
IR generation still needs to be updated to handle these implicit
conversions and downcasts. New AST nodes may follow.
Swift SVN r3459
