swift234

Start making the method overriding rules a bit more sane.

When we type check the signature of a method, determine which method
it overrides (if any) at that time. This ensures that we always have
this information for name lookup (per <rdar://problem/15932845>). 

As part of this, start to make the overriding rules a little more
sane. John has worked out more of the model here, but this patch:
  - Considers an Objective-C method an override of another Objective-C
  method if the selectors match and the type vs. instance-ness of the
  methods match. The method types are checked for consistency
  (subtyping is okay).
  - Diagnoses when a method overrides more than one method from a
  superclass, and
  - Eliminates the "cannot overload method from a superclass"
  diagnostic, which is overly pedantic and oddly limiting.

Note that we lose some amount of checking here. Specifically, we don't
have a good place to check that one has not provided two different
methods that override the same superclass method. The older code did
that (somewhat), and it's not a trivial problem to solve efficiently.

This fixes the part of <rdar://problem/15597226> that is needed for
<rdar://problem/15932845>. It still doesn't handle properties,
subscripts, and undoubtedly other issues.



Swift SVN r13108