gigi.h (standard_datatypes): Add ADT_fdesc_type and ADT_null_fdesc.

	* gigi.h (standard_datatypes): Add ADT_fdesc_type and ADT_null_fdesc.
	(fdesc_type_node): Define.
	(null_fdesc_node): Likewise.
	* decl.c (gnat_to_gnu_entity) <E_Access_Subprogram_Type>: If the target
	uses descriptors for vtables and the type comes from a dispatch table,
	return the descriptor type.
	* trans.c (Attribute_to_gnu) <Attr_Unrestricted_Access>: If the target
	uses descriptors for vtables and the type comes from a dispatch table,
	build a descriptor in the static case and copy the existing one in the
	non-static case.
	(gnat_to_gnu) <N_Null>: If the target uses descriptors for vtables and
	the type is a pointer-to-subprogram coming from a dispatch table,
	return the null descriptor.
	<N_Unchecked_Type_Conversion>: If the target uses descriptors for
	vtables, the source type is the descriptor type and the target type
	is a pointer type, first build the pointer.
	* utils.c (init_gigi_decls): If the target uses descriptors for vtables
	build the descriptor type and the null descriptor.

From-SVN: r134101