swift234

Rationalize the class-instance dynamicCast runtime functions.

dynamicCastClass assumes that the destination type is a
Swift class type.

dynamicCastObjCClass assumes that the destination type is
an ObjC class type (represented as ObjC metadata, not type
metadata).

dynamicCastUnknownClass assumes only that the destination
type is some kind of class.

Swift SVN r18776