swift234

Use -isKindOfClass: when casting to an Objective-C type.

Cocoa allows objects to override -isKindOfClass: to lie about their class.
As bad as it is, Swift should respect that. <rdar://problem/13013432>

Note that this clashes with the SWIFT_DISABLE_SWIFT_BIT_ON_CLASS_DATA hack;
on 10.9 -isKindOfClass: will always be used. In the long run, we don't
really want to ask if the target /type/ is an Objective-C type, but rather
if the target /object/ is an Objective-C object.

Swift SVN r3911