ada: Fix missing dispatching for default implementation of S'Class'{Read,Write}

Note that 'Output and 'Put_Image are also affected since they share a common
implementation with the others.

gcc/ada/ChangeLog:

	PR ada/18205
	* exp_attr.adb (Rewrite_Attribute_Proc_Call): Skip the conversion to
	the formal type only if the actual is of a class-wide type. Generate
	a dispatching call when both the prefix is a class-wide type and the
	subprogram is a dispatching operation.
	(Expand_N_Attribute_Reference) <Input>: Use U_Type consistently and
	remove an useless invocation of Root_Type.
	<Put_Image>: Minor comment tweak.
	<Read>: Likewise.
	<Write>: Likewise.