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

[Jim Blandy <jimb at redhat dot com>]

This patch still avoids building the all_comp_units table if it never
finds a CU whose abbrev table suggests it might contain inter-CU
references.  What I was actually suggesting was building
all_comp_units for every objfile, unconditionally --- even for
objfiles with no inter-CU references, and which will thus never use
the table.

The win would be to remove the case of an absent all_comp_units table
from the code.  I want to keep the number of different cases one needs
to think about down where doing so doesn't provide a noticeable
performance hit.

On an 8000-CU objfile (what you estimated glibc compiled with
duplicate elimination would have), that would be a memory overhead of
160k, which doesn't seem bad.  It's true that that would be a separate
pass over the .debug_info section, but the latest patch will do two
passes anyway.

The quadratic bit in create_all_comp_units to recover the psymtabs
we'd already created wouldn't be needed any more.