[Ada] Missing errors on aspect checking

[Ada] Missing errors on aspect checking

2020-06-15  Arnaud Charlet  <charlet@adacore.com>

gcc/ada/

	* sem_attr.adb (Eval_Attribute): Protect against previous
	errors.
	* sem_ch13.adb (Analyze_Aspect_Default_Value): Remove redundant
	error checking, handling in Analyze_Aspect_Specifications.
	(Analyze_Aspect_Specifications): Refine error messages on
	Default_[Component_]Value.
	(Check_Aspect_Too_Late): New procedure.
	(Rep_Item_Too_Late.Is_Derived_Type_With_Constraint): Remove,
	dead code.
	* aspects.ads (Is_Representation_Aspect): Default_Value is a
	representation aspect.