swift234

Perform override checking prior to (and independent of) @objc computation.

Always perform override checking based on the Swift type
signatures, rather than alternately relying on the Objective-C
selectors. This ensures that we get consistent override behavior for
@objc vs. non-@objc declarations throughout, and we separately make
sure that the Objective-C names line up.

This also allows us to inherit @objc'ness correctly (which didn't
quite work before), including inferring the Objective-C selector/name
(the actual subject of rdar://problem/18998564).

Fixes rdar://problem/18998564.

Swift SVN r25392