ada: Futher refinements to mutably tagged types

This patch further enhances the mutably tagged type implementation by fixing
several oversights relating to generic instantiations, attributes, and
type conversions.

gcc/ada/

	* exp_put_image.adb (Append_Component_Attr): Obtain the mutably
	tagged type for the component type.
	* mutably_tagged.adb (Make_Mutably_Tagged_Conversion): Add more
	cases to avoid conversion generation.
	* sem_attr.adb (Check_Put_Image_Attribute): Add mutably tagged
	type conversion.
	* sem_ch12.adb (Analyze_One_Association): Add rewrite for formal
	type declarations which are mutably tagged type to their
	equivalent type.
	(Instantiate_Type): Add condition to obtain class wide equivalent
	types.
	(Validate_Private_Type_Instance): Add check for class wide
	equivalent types which are considered "definite".
	* sem_util.adb (Is_Variable): Add condition to handle selected
	components of view conversions. Add missing check for selected
	components.
	(Is_View_Conversion): Add condition to handle class wide
	equivalent types.