Thank you so much @dblaikie for this. Admittedly, for all the direction I've been digging, missed this one :)
The catch here is that getDwoLineTable(U)->getFile(... this will enable the line table through MCDwarf interface, so we don't have/need to rely in DwarfTypeUnit interface for enabling line table.
Anyways, I'm planning to abandon all the revisions associated with this. IMHO D82084 is also not needed(since you did it without touching that) so we can leave it until there is actual need/situation ?
A side note: Do you also plan to put this in LLVM11 branch. AFAIK -gdwarf-5 -gsplit-dwarf -fdebug-macro still produces same error(Relocation in DWO object). This patch solves that.