Also, suggest "xcrun swiftc" instead of "xcrun swift" on OS X, since
"swift" already infers SDKs and we shouldn't get into this situation.
(We shouldn't get into it on non-OS-X either thanks to the previous
commit, but just in case.)
rdar://problem/22440615
Swift SVN r31535
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
