Delete `DIRef<>`, and replace the remaining uses of it with
`TypedDebugNodeRef<>`. To minimize code churn, I've added typedefs from
`MDTypeRef` to `DITypeRef` (etc.).
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@235071 91177308-0d34-0410-b5e6-96231b3b80d8