[Ada] Avoid inappropriate error messages regarding aggregates and variant parts

[Ada] Avoid inappropriate error messages regarding aggregates and variant parts

gcc/ada/

	* sem_util.adb (Gather_Components): Factor the test that was
	already being used to govern emitting a pre-Ada_2020 error
	message into an expression function,
	OK_Scope_For_Discrim_Value_Error_Messages. Call that new
	function in two places: the point where the same test was being
	performed previously, and in governing emission of a newer
	Ada_2020 error message. In both cases, the out-mode parameter
	Gather_Components.Report_Errors is set to True even if no error
	messages are generated within Gather_Components.
	* sem_util.ads: Correct a comment.