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 is needed for tests which define internal functions which should not be eliminated.
So far this was not needed because of a hack which prevented whole-module-optimizations for tests.
Swift SVN r22658