Core/Defines: define UNIT_FLAG_SWIMMING

Port From (https://github.com/TrinityCore/TrinityCore/commit/4a47e1c6be02608d8df2ec1b6267a31dcd0c41fa)

Source/Framework/Constants/CreatureConst.cs

@@ -63,7 +63,7 @@ namespace Framework.Constants
         Pvp = 0x1000,
         Silenced = 0x2000,
         CannotSwim = 0x4000,
-        Unk15 = 0x8000,
+        Swimming = 0x8000, // shows swim animation in water
         NonAttackable2 = 0x10000,
         Pacified = 0x20000,
         Stunned = 0x40000,

Source/Game/Entities/Unit/Unit.Movement.cs

@@ -61,7 +61,7 @@ namespace Game.Entities
             if (IsPet() && HasUnitFlag(UnitFlags.PetInCombat))
                 return true;
 
-            return HasUnitFlag(UnitFlags.Rename | UnitFlags.Unk15);
+            return HasUnitFlag(UnitFlags.Rename | UnitFlags.Swimming);
         }
         public virtual bool IsInWater()
         {