Lazy-allocate ValidationState.CurPos (#54346)

The array is only used for a subset of validators; no point in paying for this array for every ValidationState instance even if it's not going to be used.

src/libraries/System.Private.Xml/src/System/Xml/Schema/ValidationState.cs

@@ -42,7 +42,8 @@ internal sealed class ValidationState
         public bool HasMatched;       // whether the element has been verified correctly
 
         //For NFAs
-        public BitSet[] CurPos = new BitSet[2];
+        private BitSet[]? _curPos;
+        public BitSet[] CurPos => _curPos ??= new BitSet[2];
 
         //For all
         public BitSet? AllElementsSet;