swift234

Clang importer: clean up overriding and avoid adding implicit initializers.

Switch the "does this imported declaration override a declaration in
the superclass?" check to perform lookups based on what actually gets
into Swift, rather than looking into the Objective-C class, because
the Objective-C class isn't representative.

Additionally, explicitly avoid adding implicit initializers to an
Objective-C-imported class.

Problems fixed here are tested by an upcoming commit.

Swift SVN r16464