DebugInfo: Simplify a few more wrappers

This is almost NFC, but I'm removing some assertions against `nullptr`.
The assertions aren't worth all that much since we'll typically get
segfaults at the same site (and I imagine ASan catches this sort of
thing), and besides: the whole idea is to replace the `DIDescriptor`
hierarchy with raw pointers to the new one.

llvm-svn: 234802