Add NSMutableOrderedSet subscript setter that is missing from non-Darwin

Author: rxwei

Foundation/NSOrderedSet.swift

@@ -391,6 +391,16 @@ open class NSMutableOrderedSet : NSOrderedSet {
         _storage.remove(value)
         _orderedStorage.remove(at: index(of: object))
     }
+
+    open override subscript(idx: Int) -> Any {
+        get {
+            return object(at: idx)
+        }
+        set {
+            replaceObject(at: idx, with: newValue)
+        }
+    }
+    
 }
 
 extension NSMutableOrderedSet {

TestFoundation/TestNSOrderedSet.swift

@@ -207,10 +207,11 @@ class TestNSOrderedSet : XCTestCase {
     func test_ReplaceObject() {
         let set = NSMutableOrderedSet(arrayLiteral: "foo", "bar", "baz")
         set.replaceObject(at: 1, with: "123")
+        set[2] = "456"
         XCTAssertEqual(set.count, 3)
         XCTAssertEqual(set[0] as? String, "foo")
         XCTAssertEqual(set[1] as? String, "123")
-        XCTAssertEqual(set[2] as? String, "baz")
+        XCTAssertEqual(set[2] as? String, "456")
     }
 
     func test_ExchangeObjects() {