Use ctor rather than MemberwiseClone to guarantee a safer clone of BeatmapDifficulty

osu.Game.Rulesets.Taiko/Beatmaps/TaikoBeatmapConverter.cs

@@ -208,6 +208,8 @@ namespace osu.Game.Rulesets.Taiko.Beatmaps
 
             #region Overrides of BeatmapDifficulty
 
+            public override BeatmapDifficulty Clone() => new TaikoMultiplierAppliedDifficulty(this);
+
             public override void CopyTo(BeatmapDifficulty other)
             {
                 base.CopyTo(other);

osu.Game/Beatmaps/BeatmapDifficulty.cs

@@ -37,12 +37,7 @@ namespace osu.Game.Beatmaps
         /// <summary>
         /// Returns a shallow-clone of this <see cref="BeatmapDifficulty"/>.
         /// </summary>
-        public BeatmapDifficulty Clone()
-        {
-            var diff = (BeatmapDifficulty)MemberwiseClone();
-            CopyTo(diff);
-            return diff;
-        }
+        public virtual BeatmapDifficulty Clone() => new BeatmapDifficulty(this);
 
         public virtual void CopyTo(BeatmapDifficulty difficulty)
         {