Driver/Darwin: Set the DWARF version based on the deployment target.

System utilities such as atos only support DWARF 4 on OS X 10.11+ and
iOS 9+. We thus want to enable DWARF 4 only if the deployment target
has a recent enough operating system version and use DWARF 2 for older
systems.

<rdar://problem/28766743>

llvm-svn: 284416