swift234

Improve diagnostics for selector collisions with @objc optional requirements.

When an optional requirement of an @objc protocol has a selector that
collides with an entity that has a different *Swift* name but produces
an Objective-C method with the same selector, we have an existing
diagnostic complaining about the conflict. In such cases, make a few
suggestions (with Fix-Its) to improve the experience:

* Change Swift name to match the requirement, adding or modifying the
  @objc as appropriate.
* Add "@nonobjc" to silence the diagnostic, explicitly opting out of
  matching an @objc requirement.

This is intended to help with migration of Swift 2 code into Swift
3. The Swift 2 code will produce selectors that match Objective-C
methods in the protocol from Swift names that don't match; this helps
fix up those Swift names so that we now match.

Fixes the rest of rdar://problem/25159872. In some sense, it's a
stop-gap for more detailed checking of near-misses for optional
requirements, but it's not clear how wide-reaching such changes would
be.