This is still a subject of discussion on swift-dev, but it seems like clearly the right
way to go to me. If it turns out that this isn't a good direction, I'll revert this and
subsequent patches built on top of it.
Swift SVN r26168
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