* dwarf2read.c (struct dwarf2_per_cu_data): Move member
	type_unit_group ...
	(struct signatured_type): ... to here.
	(sig_type_ptr): New typedef.
	(type_unit_group): Delete member 't.first_cu'.  Move member 'tus'
	out of union 't'.  All uses updated.
	(dw2_get_file_names_reader): Assert not called for a type unit.
	(dw2_get_file_names): Assert not called for a type unit or type
	unit group.
	(build_type_psymtabs_reader): Assert called for a type unit.
	(build_type_psymtab_dependencies): Assert called for a type unit group.