swift234

QoI: Improve diagnostics for failed @objc inference.

When we cannot infer an @objc name due to an ambiguity, provide a
specific error with Fix-Its for various courses of action (pick a
specific name, make it @nonobjc). Also, be sure to suppress redundant
diagnostics for Objective-C selector checking when checking protocol
conformances: a match-with-renaming will emit the appropriate '@objc'
when it's needed, so we don't need a follow-up diagnostic here.

Finishes rdar://problem/26518216.