swift234

Match Objective-C names in overrides as part of validation, not override checking.

Previously, we were using the Objective-C names to help determine
whether a declaration is an override or not. This is broken, because
we should determine overrides based on the Swift rules for
overriding, then (later) check that the Objective-C runtime will see
the same override behavior that the Swift runtime does. Address this
problem, both by taking the Objective-C selector out of the equation
when matching overrides (except for diagnostic purposes) and by
performing better validation of the Objective-C names for the
overriding vs. overridden methods/properties.

The motivating case here (from rdar://problem/18998564) is an
Objective-C initializer:

  -(instancetype)initString:(NSString *)string;

When trying to override this in a Swift subclass, one naturally
writes:

  override init(string: String)

which implicitly has the selector initWithString:. We ended up in an
unfortunate place where we rejected the override (because the
selectors didn't match) with a crummy diagnostic, but omitting the
"override" would result in a different conflict with the superclass.

Now, we'll treat this as an override and complain that one needs to
rename the method by adding "@objc(initString:)" (with a Fix-It, of
course). This fixes rdar://problem/18998564, but it is not ideal: the
complete solution (covered by rdar://problem/19812955) involves
reworking the dance between override and @objc so that we compute
'override' first (ignoring @objc-ness entirely), and let the
@objc'ness of the overridden declaration both imply @objc for the
overriding declaration and implicitly fix the selector. However, such
a change is too risky right now, hence the radar clone.

Swift SVN r25243