ada: Constraint_Error caused by 'Image applied to interface type

When the prefix of 'Image is used with a class-wide interface
type object, the frontend does not generate code to displace
the pointer to the underlying object to reference its base,
and this is required to invoke Ada.Tags.Wide_Wide_Expanded_Name.

gcc/ada/
	* exp_imgv.adb (Rewrite_Object_Image): fix type of formal. Found
	reading sources.
	(Expand_Wide_Image_Attribute): ditto.
	(Expand_Wide_Wide_Image_Attribute): ditto.
	(Rewrite_Object_Image): ditto.
	* exp_put_image.adb (Build_Image_Call): For class-wide interface
	type prefix generate code to displace the pointer to the object to
	reference the base of the underlying object.