* dwarf2out.c (struct external_ref, build_local_stub): New.
	(hash_external_ref, external_ref_eq, lookup_external_ref): New.
	(optimize_external_refs, optimize_external_refs_1): New.
	(change_AT_die_ref): New.
	(clone_as_declaration): Add DW_AT_signature when cloning a declaration.
	(build_abbrev_table): Take the external refs hashtable.
	(output_comp_unit): Get it from optimize_external_refs and pass it in.

From-SVN: r187116