Use Linux sysroot in Driver test case from r234521
Use Linux sysroot in Driver test case from r234521

We don't actually need a real Mac sysroot to make the test pass, just a
linker.  This makes the test pass in environments where no ld is on
PATH.

git-svn-id: ffe668792ed300d6c2daa1f6eba2e0aa28d7ec6c@234533 91177308-0d34-0410-b5e6-96231b3b80d8

apple/swift-clang,apple/swift-clang,apple/swift-clang,llvm-mirror/clang,llvm-mirror/clang,llvm-mirror/clang,llvm-mirror/clang,llvm-mirror/clang,apple/swift-clang,llvm-mirror/clang,apple/swift-clang,llvm-mirror/clang,apple/swift-clang,llvm-mirror/clang,llvm-mirror/clang,apple/swift-clang,apple/swift-clang,apple/swift-clang,apple/swift-clang,llvm-mirror/clang