This fixes a case where the Swift-variadic and C-varargs versions of
various initializers were superseding each other
<rdar://problem/16801456>.
It also uncovered some more cases where we weren't getting quite the
right semantics for factory-methods-as-initializers, which are also
fixed here.
Swift SVN r18010
A Swift method in an extension cannot override a method nor can it be
overridden by a method. Diagnose this. We may remove this limitation
at a later time.
An [objc] method in an extension can override and can be overridden, so
specifically allow this case. It's useful in our Foundation bindings.
Swift SVN r4308
