@Deprecated should warn if you're overriding a deprecated method

[tvolkert]

Consider the following code:

class Foo {
  @protected
  @Deprecated('Do not override this; override betterFoo() instead.')
  void foo() {}

  @protected
  void betterFoo() {}
}

class Bar extends Foo {
  @override
  void foo() {}
}

The fact that Bar is overriding a deprecated method does not trigger an analyzer hint code (the closest hint code that exists is DEPRECATED_MEMBER_USE, but that's for call-sites, not override-sites). This makes it almost impossible for classes to deprecate protected APIs and get subclasses to migrate away from extending the deprecated API.

We should add a new hint code, DEPRECATED_OVERRIDE, to cover this case.