Fix System.Collections.Immutable!ImmutableArray<T>.Builder.Sort() to …

…not generate Comparer wrapper each time

System.Collections.Immutable!ImmutableArray<T>.Builder.Sort() (3 overloads) creates a new Comparer wrapper each time. This can get expensive if you call Sort a lot.

This fix optmizes method Sort() alone among the 3 overloads. Sort() uses the default comparer which through this fix is getting cached into a static field in type Comparer such that the Comparer wrapper is not created each time Sort() is called.
Existing testcase Sort(), covers this case.

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

@@ -590,7 +590,7 @@ public void ReverseContents()
             /// </summary>
             public void Sort()
             {
-                Array.Sort(this.elements, 0, this.Count, new Comparer(Comparer<T>.Default));
+                Array.Sort(this.elements, 0, this.Count, Comparer.Default);
             }
 
             /// <summary>
@@ -702,6 +702,8 @@ private sealed class Comparer : IComparer<RefAsValueType<T>>
             {
                 private readonly IComparer<T> comparer;
 
+                public static readonly Comparer Default = new Comparer(Comparer<T>.Default);
+
                 internal Comparer(IComparer<T> comparer = null)
                 {
                     Requires.NotNull(comparer, "comparer");