Debug Info: set default to gdwarf-2 for Darwin.

Darwin systems currently do not support dwarf version 3 or above. When we are
ready, we can bump the default to gdwarf-4 for Darwin.

For other systems, the default is dwarf version 3, if everything goes smoothly,
we can bump the version to 4.

rdar://13591116

llvm-svn: 185483