swift234

Remove reverse subtype constraint between Objective-C classes when type
checking checked cast via bridging. It prevented bridging upcasts using
'as!' from typechecking; we should emit an 'as!'->'as' warning instead.

Also, use ExplicitConversion constraint instead of Conversion when
determining whether a checked cast can be carried out unconditionally.
This matches the constraint used after applying the 'as!'->'as' fixit.

<rdar://problem/19813772>

Swift SVN r28028