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


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@185483 91177308-0d34-0410-b5e6-96231b3b80d8