Most tests were using %swift or similar substitutions, which did not
include the target triple and SDK. The driver was defaulting to the
host OS. Thus, we could not run the tests when the standard library was
not built for OS X.
Swift SVN r24504
This doesn't really affect anyone in real life, but it will catch cases
where someone's trying to compile for iOS without the iOS stdlib around.
Swift SVN r18796
