Push memory ownership of DwarfUnits into clients of DwarfFile.

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...

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@206925 91177308-0d34-0410-b5e6-96231b3b80d8