Be more strict about what kinds of types can be passed.

Be more strict about what kinds of types can be passed.

gdb/ChangeLog:

	* gnu-v3-abi.c (gnuv3_dynamic_class): Assert only passed structs
	or unions.  Return zero if union.
	(gnuv3_get_vtable): Call check_typedef.  Assert only passed structs.
	(gnuv3_rtti_type): Pass already-check_typedef'd value to
	gnuv3_get_vtable.
	(compute_vtable_size): Assert only passed structs.
	(gnuv3_print_vtable): Don't call gnuv3_get_vtable for non-structs.