API Notes File: Add the notes generated as the product of the first nullability audit of Foundation

As well as the required test modifications.

Swift SVN r21126

test/stdlib/NSStringAPI.swift

@@ -13,7 +13,7 @@ import Foundation
 // The most simple subclass of NSString that CoreFoundation does not know
 // about.
 class NonContiguousNSString : NSString {
-  required init(coder aDecoder: NSCoder!) {
+  required init(coder aDecoder: NSCoder) {
     fatalError("don't call this initializer")
   }