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
