Previously the .debug_line.dwo section was only emitted only if type
units are present. This patch further extends emission for the case if
.debug_macro.dwo section is present.
For enabling this, I've refactored out SplitLineTable from
DwarfTypeUnit to DwarfUnit so that it can be emitted for when
type units are not present. Emission is further restricted to only
when .debug_macro.dwo section is present.
Ideally, this should've splitted into more than one patches, but
that can be done post review. This patch is intended to convey the overall