Set default Dwarf Version for -gline-tables-only on Darwin to 2.

We are still using Dwarf Version 2 for Darwin systems, make it consistent
with -gline-tables-only.

This should fix an internal buildbot.


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@195267 91177308-0d34-0410-b5e6-96231b3b80d8