[PATCH v2 3/4] Handle simple inheritance in gengtype.

[Jeff Law <law at redhat dot com>]

Treat GTY structs that have a "desc" as being the root of an inheritance
hierarchy.  Generate a switch on desc within the marking function with
cases for each subclass, visiting all fields of the type (including
inherited ones).

Don't create marking functions for subclasses, instead using the base
class marking functions.  Use walk_type on them within walk_subclasses
to generate the case within the switch for handling the tag, directly
walking all fields of the type.

	* 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.
This is OK.

Jeff