Mirror changes from dotnet/corefx

src/System.Private.CoreLib/shared/System/Text/ValueStringBuilder.cs

@@ -22,6 +22,13 @@ public ValueStringBuilder(Span<char> initialBuffer)
             _pos = 0;
         }
 
+        public ValueStringBuilder(int initialCapacity)
+        {
+            _arrayToReturnToPool = ArrayPool<char>.Shared.Rent(initialCapacity);
+            _chars = _arrayToReturnToPool;
+            _pos = 0;
+        }
+
         public int Length
         {
             get => _pos;