Core/Maps: Add override to allow InstanceMap const* -> InstanceScript const*.

Also tighten const-ness on CanSpawn method of CreatureScript.

src/server/game/Scripting/ScriptMgr.h

@@ -434,7 +434,7 @@ class TC_GAME_API CreatureScript : public UnitScript, public UpdatableScript<Cre
         virtual uint32 GetDialogStatus(Player* /*player*/, Creature* /*creature*/) { return DIALOG_STATUS_SCRIPTED_NO_STATUS; }
 
         // Called when the creature tries to spawn. Return false to block spawn and re-evaluate on next tick.
-        virtual bool CanSpawn(ObjectGuid::LowType /*spawnId*/, CreatureTemplate const* /*cTemplate*/, CreatureData const* /*cData*/, Map const* /*map*/) { return true; }
+        virtual bool CanSpawn(ObjectGuid::LowType /*spawnId*/, CreatureTemplate const* /*cTemplate*/, CreatureData const* /*cData*/, Map const* /*map*/) const { return true; }
 
         // Called when a CreatureAI object is needed for the creature.
         virtual CreatureAI* GetAI(Creature* /*creature*/) const { return NULL; }