The Driver/linker-clang_rt test had some of its run lines with incorrect prefixes.
(For example, the prefix for the iOS tests was 'IOS' rather than 'CHECK-IOS'). The
effect is that the CHECK lines in the test weren't actually used and so little was
being tested.
The commits updates the --prefix arguments so that the CHECK lines are active.
This also updates the test to specify the current behavior: namely that the
compiler-rt path is in "/lib/swift/clang/lib/darwin/" rather than "/lib/clang/darwin/".
I've also updated one of the tvOS lines to use the "tvos" in the target rather than
"ios". I suspect that was typo that wasn't caught the test wasn't running.
