swift234

Require witnesses for @objc requirements to be @objc.

Previously, we attempted to infer @objc-ness based on conformance, but
doing so is fraught with ordering dependencies, and just doesn't work
in the general case. Among other crimes, this allowed us to
retroactively mark a non-@objc method from an imported module as
@objc... even though nobody would ever then emit the @objc entry
points for it.

Fixes the rest of rdar://problem/18383574.

Swift SVN r24831