DwarfUnit: Replace unnecessary conditionals with asserts.

We used to be pretty vague about what debug entities were what, with
many conditionals to silently drop/skip/accept things. These don't seem
to be relevant anymore.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@201194 91177308-0d34-0410-b5e6-96231b3b80d8