DIBuilder: Use Constant instead of Value

Make explicit the requirement that most IR values in `DIBuilder` are
`Constant`.  This requires a follow-up change in clang.

Part of PR21532.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@222070 91177308-0d34-0410-b5e6-96231b3b80d8