Use consistent header for AutoDiff.{h,cpp}.

Author: dan-zheng

include/swift/AST/AutoDiff.h

@@ -1,4 +1,4 @@
-//===--- AutoDiff.h - Swift Automatic Differentiation ---------------------===//
+//===--- AutoDiff.h - Swift automatic differentiation utilities -----------===//
 //
 // This source file is part of the Swift.org open source project
 //
@@ -10,7 +10,7 @@
 //
 //===----------------------------------------------------------------------===//
 //
-//  This file defines AST support for automatic differentiation.
+//  This file defines utilities for automatic differentiation.
 //
 //===----------------------------------------------------------------------===//
 

lib/AST/AutoDiff.cpp

@@ -1,8 +1,8 @@
-//===--------- AutoDiff.cpp - Swift Differentiable Programming ------------===//
+//===--- AutoDiff.cpp - Swift automatic differentiation utilities ---------===//
 //
 // This source file is part of the Swift.org open source project
 //
-// Copyright (c) 2014 - 2017 Apple Inc. and the Swift project authors
+// Copyright (c) 2019 Apple Inc. and the Swift project authors
 // Licensed under Apache License v2.0 with Runtime Library Exception
 //
 // See https://swift.org/LICENSE.txt for license information