* dwarf2read.c (queue_and_load_all_dwo_tus): New function.

	* dwarf2read.c (queue_and_load_all_dwo_tus): New function.
	(queue_and_load_dwo_tu): New function.
	(lookup_dwo_signatured_type): Set per_cu.tu_read.
	(maybe_queue_comp_unit): Rename this_cu argument to dependent_cu.
	Make dependent_cu optional.
	(dw2_do_instantiate_symtab): If we just loaded a CU from a DWO,
	and an older .gdb_index is in use, queue and load all its TUs too.

	testsuite/
	* gdb.base/enumval.c (ZERO): New enum value.
	(main): Use it
	* gdb.base/enumval.exp: Test ability to print ZERO.