Core/Scripts port _DespawnAtEvade functionality to 3.3.5 branch

Implement _DespawnAtEvade  functionality to boss Lich King

src/server/game/AI/ScriptedAI/ScriptedCreature.cpp

@@ -547,6 +547,29 @@ void BossAI::UpdateAI(uint32 diff)
     DoMeleeAttackIfReady();
 }
 
+void BossAI::_DespawnAtEvade(uint32 delayToRespawn)
+{
+    if (delayToRespawn < 2)
+    {
+        TC_LOG_ERROR("scripts", "_DespawnAtEvade called with delay of %u seconds, defaulting to 2.", delayToRespawn);
+        delayToRespawn = 2;
+    }
+
+    uint32 corpseDelay = me->GetCorpseDelay();
+    uint32 respawnDelay = me->GetRespawnDelay();
+
+    me->SetCorpseDelay(1);
+    me->SetRespawnDelay(delayToRespawn - 1);
+
+    me->DespawnOrUnsummon();
+
+    me->SetCorpseDelay(corpseDelay);
+    me->SetRespawnDelay(respawnDelay);
+
+    if(instance)
+        instance->SetBossState(_bossId, FAIL);
+}
+
 // WorldBossAI - for non-instanced bosses
 
 WorldBossAI::WorldBossAI(Creature* creature) :