Revert "Debug info: Make DWARF4 the default for Darwin, too."

I'm holding this change to give maintainers of Darwin buildbots more time
to update their toolchains.

This reverts commit r201375.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@201520 91177308-0d34-0410-b5e6-96231b3b80d8