[Ada] AI12-0042: Type invariant checking rules

[Ada] AI12-0042: Type invariant checking rules

gcc/ada/

	* exp_ch4.adb (Expand_N_Type_Conversion): Handle the case of
	applying an invariant check for a conversion to a class-wide
	type whose root type has a type invariant, when the conversion
	appears within the immediate scope of the type and the
	expression is of a specific tagged type.
	* sem_ch3.adb (Is_Private_Primitive): New function to determine
	whether a primitive subprogram is a private operation.
	(Check_Abstract_Overriding): Enforce the restriction imposed by
	AI12-0042 of requiring overriding of an inherited nonabstract
	private operation when the ancestor has a class-wide type
	invariant and the ancestor's private operation is visible.
	(Derive_Subprogram): Set Requires_Overriding on a subprogram
	inherited from a visible private operation of an ancestor to
	which a Type_Invariant'Class expression applies.