[Ada] Avoid building malformed component constraints

[Ada] Avoid building malformed component constraints

gcc/ada/

	* sem_util.adb (Build_Actual_Subtype_Of_Component): Define a new
	local function, Build_Discriminant_Reference, and call it in
	each of the three cases where Make_Selected_Component was
	previously being called to construct a discriminant reference (2
	in Build_Actual_Array_Constraint and 1 in
	Build_Actual_Record_Constraint). Instead of unconditionally
	using the passed-in object name as the prefix for the new
	selected component node, this new function checks to see if
	perhaps a prefix of that name should be used instead.