Core/Loot: Set loot dungeon encounter id before generating

Port From (https://github.com/TrinityCore/TrinityCore/commit/5f1a4cf9044d9ad65b49eb859ec36e1f62c2be10)

Source/Game/Maps/Instances/InstanceScript.cs

@@ -16,18 +16,15 @@
  */
 
 using Framework.Constants;
-using Framework.Database;
-using Framework.IO;
+using Game.AI;
 using Game.DataStorage;
 using Game.Entities;
 using Game.Groups;
 using Game.Networking.Packets;
-using Game.Scenarios;
 using Game.Spells;
 using System;
 using System.Collections.Generic;
 using System.Linq;
-using System.Text;
 
 namespace Game.Maps
 {
@@ -703,6 +700,15 @@ namespace Game.Maps
         {
             return id < bosses.Count ? bosses[id].GetDungeonEncounterForDifficulty(instance.GetDifficultyID()) : null;
         }
+
+        public DungeonEncounterRecord GetBossDungeonEncounter(Creature creature)
+        {
+            BossAI bossAi = (BossAI)creature.GetAI();
+            if (bossAi != null)
+                return GetBossDungeonEncounter(bossAi.GetBossId());
+
+            return null;
+        }
         
         public virtual bool CheckAchievementCriteriaMeet(uint criteria_id, Player source, Unit target = null, uint miscvalue1 = 0)
         {