swift234

Sema: Differentiate between 'class is @objc' and 'class has implicitly @objc members'

Generic subclasses of @objc classes are thus no longer @objc, but still
have implicitly @objc members.

Explicit @objc on generic classes or classes that inherit from @objc
classes is now forbidden with a diagnostic. Users need to know that
while they can override Objective-C methods and properties in such
a class, they cannot refer to the class by name from Objective-C code,
since it will not appear in the bridging header.

Fixes <rdar://problem/21342574>.

Swift SVN r30494