swift234

Disallow @objc on non-ObjC-rooted classes.

These classes don't show up well in generated headers (rdar://problem/20855568),
can't actually be allocated from Objective-C (rdar://problem/17184317), and
make the story of "what is exposed to Objective-C" more complicated. Better
to just disallow them.

All classes are still "id-compatible" in that they can be converted to
AnyObject and passed to Objective-C, they secretly implement NSObjectProtocol
(via our SwiftObject root class), and their members can still be individually
exposed to Objective-C.

The frontend flag -disable-objc-attr-requires-foundation-module will disable
this requirement as well, which is still necessary for both the standard
library and a variety of tests I didn't feel like transforming.

Swift SVN r29760