The 'override' attribute indicates that the given declaration, which
may be a method, property, or subscript, overrides a declaration in
its superclass. Per today's discussion, the 'override' attribute must
be present if and only if the corresponding declaration overrides a
declaration in its superclass.
This implements most of <rdar://problem/14798539>. There's still more
work to do to on property and subscript overrides.
Swift SVN r14388
