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
This prevents us from seeing a less useful error message from SILGen
further down the line.
Also fix a bug where @objc without importing Foundation was not diagnosed
after the first top-level form. Some tests were relying on this behavior,
so fix those tests, either by splitting off the objc parts of the test, or
just by passing the -disable-objc-attr-requires-foundation-module flag.
Fixes <rdar://problem/20660270>.
Swift SVN r29359
