swift234

Allow 'T?' to override '@unchecked T?' in both parameter and return position.

Also, allow 'Int?' to override 'Int?' (checked or unchecked); previously only
class overrides were allowed.

T? is clearly a supertype of @unchecked T?, since the rules about how T?
can be used are stricter. On the other hand, T? communicates more information
to a user of a method than @unchecked T?, while not actually changing the
range of possible values. The end result will be that someone calling 'foo()'
through the base class will get an @unchecked T?, while someone calling
'foo()' through the subclass will get a normal, checked T?. That seems
reasonable, particularly because it allows overriding any Objective-C methods
without actually uttering "@unchecked".

Swift SVN r14606