ada: Small cleanup in processing of primitive operations

ada: Small cleanup in processing of primitive operations

The processing of primitive operations is now always uniform for tagged and
untagged types, but the code contains left-overs from the time where it was
specific to tagged types, in particular for the handling of subtypes.

gcc/ada/

	* einfo.ads (Direct_Primitive_Operations): Mention concurrent types
	as well as GNAT extensions instead of implementation details.
	(Primitive_Operations): Document that Direct_Primitive_Operations is
	also used for concurrent types as a fallback.
	* einfo-utils.adb (Primitive_Operations): Tweak formatting.
	* exp_util.ads (Find_Prim_Op): Adjust description.
	* exp_util.adb (Make_Subtype_From_Expr): In the private case with
	unknown discriminants, always copy Direct_Primitive_Operations and
	do not overwrite the Class_Wide_Type of the expression's base type.
	* sem_ch3.adb (Analyze_Incomplete_Type_Decl): Tweak comment.
	(Analyze_Subtype_Declaration): Remove older and now dead calls to
	Set_Direct_Primitive_Operations.  Tweak comment.
	(Build_Derived_Private_Type): Likewise.
	(Build_Derived_Record_Type): Likewise.
	(Build_Discriminated_Subtype): Set Direct_Primitive_Operations in
	all cases instead of just for tagged types.
	(Complete_Private_Subtype): Likewise.
	(Derived_Type_Declaration): Tweak comment.
	* sem_ch4.ads (Try_Object_Operation): Adjust description.