Reapply "NFC: DebugInfo: Refactor RangeSpanList to be a struct, like DebugLocStream::List"

Move these data structures closer together so their emission code can
eventually share more of its implementation.

Was an egregious bug (completely untested, evidently) where I hadn't
inverted a DWARFv5 test as needed, so it was doing the exact opposite of
what was required & thus tried to emit a DWARFv5 range list header in
DWARFv4.

Reapply 8e048962 which was
reverted in a8154e5e.