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.

(Also, fix the error that was responsible for breaking
the expr/cast/bridged.swift test.)

<rdar://problem/19813772>

Swift SVN r28034