[Ada] Implement AI12-0351 Matching for actuals for formal derived types

[Ada] Implement AI12-0351 Matching for actuals for formal derived types

2020-06-16  Eric Botcazou  <ebotcazou@adacore.com>

gcc/ada/

	* sem_ch12.adb (Validate_Derived_Type_Instance): Reword error
	message for 12.5.1(8) subclause and add secondary message if
	the incompatibility comes from the predicates.
	* sem_ch3.adb (Check_Constraining_Discriminant): New procedure
	to give the error required by the 3.7(15) subclause.  Mention
	"statically" in the error message and add secondary message
	if the incompatibility comes from the predicates.
	(Build_Derived_Concurrent_Type): Call it when a new discriminant
	constrains an old one.
	(Build_Derived_Record_Type): Likewise.
	* sem_eval.ads (Predicates_Compatible): Declare.
	* sem_eval.adb (Predicates_Compatible): New function to implement
	the compatibility of predicates specified by the 4.9.1 clause.
	(Subtypes_Statically_Compatible): Call it.