[Ada] Missing check on private overriding of dispatching primitive

[Ada] Missing check on private overriding of dispatching primitive

2020-06-09  Javier Miranda  <miranda@adacore.com>

gcc/ada/

	* sem_ch6.adb (New_Overloaded_Entity): Add missing call to check
	subtype conformance of overriding dispatching primitive.
	* sem_eval.adb (Subtypes_Statically_Match): Handle derivations
	of private subtypes.
	* libgnat/g-exptty.adb, libgnat/g-exptty.ads
	(Set_Up_Communications): Fix the profile since null-exclusion is
	missing in the access type formals.
	* sem_disp.ads (Check_Operation_From_Private_View): Adding
	documentation.