One minor revision: this lifts the proposed restriction against
overriding a non-open method with an open one. On reflection,
that was inconsistent with the existing rule permitting non-public
methods to be overridden with public ones. The restriction on
subclassing a non-open class with an open class remains, and is
in fact consistent with the existing access rule.
Include a mapping from Objective-C selectors to the @objc methods that
produce Objective-c methods with those selectors. Use this to lazily
populate the Objective-C method lookup tables in each class. This makes
@objc override checking work across Swift modules, which is part of
rdar://problem/18391046.
Note that we use a single, unified selector table, both because it is
simpler and because it makes global queries ("is there any method with
the given selector?") easier.
Swift SVN r23214
