Use visitors for make_gdb_type

Use visitors for make_gdb_type

Remove the make_gdb_type functions from the tdesc_type_ classes.
Replace with a static make_gdb_type function that uses a element
visitor called gdb_type_creator.

gdb/
	* target-descriptions.c (tdesc_element_visitor) Add empty implementations.
	(tdesc_type): Move make_gdb_type from here.
	(tdesc_type_builtin): Likewise.
	(tdesc_type_vector): Likewise.
	(tdesc_type_with_fields): Move make_gdb_type_ functions from here.
	(make_gdb_type_struct): Move from tdesc_type_with_fields.
	(make_gdb_type_union): Likewise.
	(make_gdb_type_flags): Likewise.
	(make_gdb_type_enum): Likewise.
	(make_gdb_type): New function.
	(tdesc_register_type): Use static make_gdb_type.