swift234

[PrintAsObjC] Don't print nested classes as nested in Objective-C.

There are still problems with nested classes:
- They're much more likely to have colliding compile-time names
  (since the outer class's name is dropped).
- They're only picked up if the outer class is also @objc.

But at least now we won't generate invalid Objective-C. Unless the inner
classes have the same name.

rdar://problem/18187877

Swift SVN r21677