swift234

Fix definition of "nil": it has to be null, not a default-initialized NSObject.

This ended up being easiest with a runtime function. I couldn't push this
into the standard library because the standard library doesn't know about
NSObject, and I couldn't compile the ObjectiveC module as -parse-stdlib
because it uses standard library types and doesn't otherwise know how to
find the "swift" module.

Swift SVN r7581