Make gengtype more robust against user error

Make gengtype more robust against user error

	* doc/gty.texi ("Inheritance and GTY"): Make it clear that
	to use autogenerated markers for a class-hierarchy, every class
	must have a GTY marker.
	* gengtype.h (struct type): Add linked list of subclasses to
	the "s" member of the union.
	(add_subclass): New decl.
	* gengtype-state.c (read_state_struct_type): Set up subclass
	linked list.
	* gengtype.c (get_ultimate_base_class): New.
	(add_subclass): New.
	(new_structure): Set up subclass linked list.
	(set_gc_used_type): Propagate usage information to subclasses.
	(output_mangled_typename): Use get_ultimate_base_class.
	(walk_subclasses): Use the subclass linked list, avoiding an
	O(N^2) when writing out all types.
	(walk_type): Issue an error if the base class is missing a tag,
	rather than generating bogus C code.  Add a gcc_unreachable
	default case, in case people omit tags from concrete subclasses,
	or get the values wrong.
	(write_func_for_structure): Issue an error for subclasses for
	which the base doesn't have a "desc", since otherwise the
	autogenerated routines for the base would silently fail to visit
	any subclass fields.
	(write_root): Use get_ultimate_base_class, tweaking constness of
	tp to match that function's signature.

From-SVN: r204148