test suite: on Darwin, default an SDKROOT to macosx, and provide via isysroot.
test suite: on Darwin, default an SDKROOT to macosx, and provide via isysroot.

rdar://problem/23556268

apple/swift-lldb,apple/swift-lldb,apple/swift-lldb,apple/swift-lldb,apple/swift-lldb,apple/swift-lldb