Push memory ownership of DwarfUnits into clients of DwarfFile.

This prompted me to push references through most of DwarfDebug. Sorry
for the churn.

Honestly it's a bit silly that we're passing around units all over the
place like that anyway and I think it's mostly due to the DIE attribute
adding utility functions being utilities in DwarfUnit. I should have
another go at moving them out of DwarfUnit...