Split test/Driver/darwin-sdkroot.c into two tests

The test additions in r329110 are Darwin-specific, as they rely
on a code path that is reachabled when driver is invoked without
-target. Instead of making the old test checks Darwin-specific too,
let's simply split it into two files to ensure that the old
checks are still platform-agnostic. Thanks Chandler for
suggesting this!

llvm-svn: 329141