swift234

Tweak diagnostics for non-self-conforming protocols.

Before:
- protocol type 'Listener' does not conform to protocol 'Listener' because
  'Listener' is not declared @objc
- protocol type 'Listener' does not conform to protocol 'Listener' because
  'Listener' defines static methods

After:
- using 'Listener' as a concrete type conforming to protocol 'Listener' is
  not supported
- 'Listener' cannot be used as a type conforming to protocol 'Listener'
  because 'Listener' has static requirements

I removed the mention of '@objc' even though @objc protocols are more freely
self-conforming because it was confusing people working with pure Swift code.
Making this actually work for pure Swift protocols is tracked by
rdar://problem/21341337.

This also fixes a few cases where we were emitting this message even for
two completely unrelated protocols.

rdar://problem/21525618

Swift SVN r30698