`DL` might be null, so check for that before using accessors. A WIP
patch to make `DIDescriptors` more strict fails otherwise.
As a bonus, I think the logic is easier to follow now (despite the extra
nesting depth).
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@232836 91177308-0d34-0410-b5e6-96231b3b80d8