[rfa/dwarf/doc] Inter-compilation-unit reference support for partial DIEs

[Jim Blandy]

When looking up references to dies in other compilation units, the
patch asserts a few times that the offset is valid.  I have in mind
the gdb_assert in find_partial_die, and the second gdb_assert in
dwarf2_find_containing_comp_unit.

But if the offset is wrong, that doesn't indicate an internal
inconsistency in GDB; it indicates an error in the debugging info.  So
these should be complaints or perhaps errors, but not internal errors.

(The other gdb_asserts in dwarf2_build_psymtabs_hard and
dwarf2_find_containing_comp_unit are cross-checking offsets in the
CU tree against other offsets created by reading the same data, so
those look right.)

I have to admit, there isn't much support for robust error handling in
the Dwarf reader.  I don't want to hold up this patch pending the
creation of that support, but we should at least do better than an
internal error.