Further improve warnings for relocations referring to discarded sections.

Relocations referring to discarded sections are now treated as errors
instead of warnings.

Also with this patch, we will now print the section group signature and the
object file with the prevailing definition of that group along with the
name of the symbol that the relocation is referring to. This additional
information should be much more useful to anyone trying to track down
the source of such errors.

To do so, we now map each discarded section to the Kept_section info in
the Layout class, and defer the logic that maps a discarded section to
its counterpart in the kept group. This gives us the information we need
to identify the signature symbol given the discarded section, and the
name of the object file that provided the prevailing (i.e., first)
definition of that group.

gold/
	* object.cc (Sized_relobj_file::include_section_group): Store
	reference to Kept_section info for discarded comdat sections
	regardless of size.  Move size checking to map_to_kept_section.
	(Sized_relobj_file::include_linkonce_section): Likewise.
	(Sized_relobj_file::map_to_kept_section): Add section name parameter.
	Insert size checking logic from above functions.
	(Sized_relobj_file::find_kept_section_object): New method.
	(Sized_relobj_file::get_symbol_name): New method.
	* object.h (Sized_relobj_file::map_to_kept_section): Add section_name
	parameter.  Adjust all callers.
	(Sized_relobj_file::find_kept_section_object): New method.
	(Sized_relobj_file::get_symbol_name): New method.
	(Sized_relobj_file::Kept_comdat_section): Replace object and shndx
	fields with sh_size, kept_section, symndx, and is_comdat fields.
	(Sized_relobj_file::set_kept_comdat_section): Replace kept_object
	and kept_shndx parameters with is_comdat, symndx, sh_size, and
	kept_section.
	(Sized_relobj_file::get_kept_comdat_section): Likewise.
	* target-reloc.h (enum Comdat_behavior): Change CB_WARNING to CB_ERROR.
	Adjust all references.
	(issue_undefined_symbol_error): New function template.
	(relocate_section): Pass section name to map_to_kept_section.
	Move discarded section code to new function above.
	* aarch64.cc (Target_aarch64::scan_reloc_section_for_stubs): Move
	declaration for gsym out one level.  Call issue_discarded_error.
	* arm.cc (Target_arm::scan_reloc_section_for_stubs): Likewise.
	* powerpc.cc (Relocate_comdat_behavior): Change CB_WARNING to CB_ERROR.

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