Steal one bit from DIERef's die_offset. That will make DWO's maximum CU size 4GB->2GB which should be OK. It will unfortunately also limit non-DWO DWARF size 4GB->2GB. That should be fixed by D61482 (then this patch will to steal the bit from cu_index/cu_offset).
The whole patchset available for testing as: git clone -b debugtypes git://git.jankratochvil.net/lldb