Revert "Update the version of dwarf we say we're emitting to at least 3."
temporarily while we work on plumbing through some changes to continue
supporting gdb on darwin.

This reverts commit r179122.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@179222 91177308-0d34-0410-b5e6-96231b3b80d8