rdar://problem/17198298
- Allow 'static' in protocol property and func requirements, but not 'class'.
- Allow 'static' methods in classes - they are 'class final'.
- Only allow 'class' methods in classes (or extensions of classes)
- Remove now unneeded diagnostics related to finding 'static' in previously banned places.
- Update relevant diagnostics to make the new rules clear.
Swift SVN r24260
This only tackles the protocol case (<rdar://problem/17510790>); it
does not yet generalize to an arbitrary "class" requirement on either
existentials or generics.
Swift SVN r19896
