gold assert after "relocation refers to discarded section" warnings

[Cary Coutant <ccoutant at google dot com>]

Ian,

When we see a reference to a local symbol in a discarded COMDAT
section, we print a warning "relocation refers to discarded section",
and continue. Unfortunately, if the relocation happens to be one of
the GOT relocations, we fail an assert later in Relocate::relocate()
because no GOT offset has been assigned:

  // Get the GOT offset if needed.
  // The GOT pointer points to the end of the GOT section.
  // We need to subtract the size of the GOT section to get
  // the actual offset to use in the relocation.
  bool have_got_offset = false;
  unsigned int got_offset = 0;
  switch (r_type)
    {
    case elfcpp::R_X86_64_GOT32:
    case elfcpp::R_X86_64_GOT64:
    case elfcpp::R_X86_64_GOTPLT64:
    case elfcpp::R_X86_64_GOTPCREL:
    case elfcpp::R_X86_64_GOTPCREL64:
      if (gsym != NULL)
        {
          gold_assert(gsym->has_got_offset(GOT_TYPE_STANDARD));
          got_offset = gsym->got_offset(GOT_TYPE_STANDARD) - target->got_size();
        }
      else
        {
          unsigned int r_sym = elfcpp::elf_r_sym<64>(rela.get_r_info());
          gold_assert(object->local_has_got_offset(r_sym, GOT_TYPE_STANDARD));
          got_offset = (object->local_got_offset(r_sym, GOT_TYPE_STANDARD)
                        - target->got_size());
        }
      have_got_offset = true;
      break;

    default:
      break;
    }

We could soften the assert to accept this case, but should we continue
with a got_offset of 0? Alternatively, we could print a fatal error
message at this point, or we could just turn the earlier warning into
an error. What do you think?

-cary