Handle simple inheritance in gengtype

Handle simple inheritance in gengtype

	* gengtype-parse.c (opts_have): Drop "static" so that
	we can use this from gengtype.c.
	* gengtype.c (set_gc_used_type): Mark any base class as used;
	update field traversal to visit inherited fields.
	(output_mangled_typename):  Convert references to classes within
	an inheritance hierarchy to reference the ultimate base class,
	since only it will have gt_ functions.
	(get_string_option): New.
	(walk_subclasses): New.
	(walk_type): Treat GTY structs that have a "desc" as being the
	root of an inheritance hierarchy.  Generate a switch on it
	within the marking function which walks all subclasses, adding
	cases for them via walk_subclasses.  For subclasses, visit all
	fields of the type (including inherited ones).
	(write_func_for_structure): Don't write fns for subclasses, only
	for the ultimate base class within an inheritance hierarchy.
	Subclasses-marking will be handled by the base class marking
	functions.
	(write_types): Likewise.
	(write_local_func_for_structure): Likewise.
	(USED_BY_TYPED_GC_P): Emit allocators for subclasses that have
	a "tag" option (and are thus concrete subclasses).
	(write_root): Use the marker function for the ultimate base class.
	* gengtype.h (FOR_ALL_INHERITED_FIELDS): New.
	(opts_have): Add declaration.

From-SVN: r204146