Revert "Follow-up to r205973: change the return type to const MDNode*."

This reverts commit r205974, it turns out that this wasn't such a great idea
after all. Using DIVariable as return value is self-documenting and marginally
more type safe.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@205979 91177308-0d34-0410-b5e6-96231b3b80d8