Don't use Array.Reverse in ImmutableArray<T>.Builder.Reverse

The non-generic Array.Reverse does not perform well for non-primitive
value types.

Author: justinvp

src/System.Collections.Immutable/src/System/Collections/Immutable/ImmutableArray`1+Builder.cs

@@ -624,7 +624,21 @@ public int LastIndexOf(T item, int startIndex, int count, IEqualityComparer<T> e
             /// </summary>
             public void Reverse()
             {
-                Array.Reverse(_elements, 0, _count);
+                // The non-generic Array.Reverse is not used because it does not perform
+                // well for non-primitive value types.
+                // If/when a generic Array.Reverse<T> becomes available, the below code
+                // can be deleted and replaced with a call to Array.Reverse<T>.
+                int i = 0;
+                int j = _count - 1;
+                T[] array = _elements;
+                while (i < j)
+                {
+                    T temp = array[i];
+                    array[i] = array[j];
+                    array[j] = temp;
+                    i++;
+                    j--;
+                }
             }
 
             /// <summary>