[rfa/dwarf] Support for attributes pointing to a different CU

[Jim Blandy]

Here's a entirely separate thought that shouldn't be addressed as part
of this patch, if it's even a good idea:

I wonder how much of a speed hit it would actually be to get rid of
die->type altogether, and always go through the hash table.  I suppose
that's three distinct areas of memory getting hit (die; hash table;
type) instead of just two.  But it'd be nice to get rid of the hair of
restoring the die types when we re-load a CU.  We've got two
representations of the same information, with different lifetimes, and
we're paying for the usual hair of keeping them in sync; it'd be nice
to just get rid of the more volatile one altogether.