var/let bindings to _ when they are never used, and use some values that
are only written. This is a testsuite cleanup, NFC. More to come.
Swift SVN r28406
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