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
I'm not sure how this ever worked -- it was previously always parsing the
OS X standard library but with the current target's triple.
Should fix the buildbot.
Swift SVN r24472
