DebugInfo: Reimplement DIRef<>::resolve() using TypedDebugNodeRef<>

Gut `DIRef<>::resolve()`, reimplementing it using
`TypedDebugNodeRef<>::resolve()`.  Use three separate functions rather
than some sort of type traits, since the latter (i.e., mapping `DIScope`
=> `MDScope`) seems heavy-handed.  I don't expect `DIRef<>` to last much
longer in tree anyway.

As a drive-by fix, make `TypedDebugNodeRef<>::resolve()` do the right
thing with `nullptr`.

llvm-svn: 234248