Driver: Fix a -Wshadow issue from r237091

The MachO toolchain has an isTargetIOSBased method, but it isn't
virtual so it isn't very meaningful to call it. After thinking about
this, I guess that putting this logic in the MachO class is a bit of a
layering violation anyway. Do this more like how we handle
AddLinkRuntimeLibArgs instead.

llvm-svn: 237095