[AutoDiff] Add @differentiable fixit for protocols/classes.

[dan-zheng]

For protocol requirements and class members with @differentiable attribute,
conforming types and subclasses must have the same @differentiable attribute
(or one with a superset of differentiability parameters) on implementing/
overriding declarations.

For implementing/overriding declarations that are missing a @differentiable
attribute, emit a fixit that adds the missing attribute.

Resolves TF-1118.

This @differentiable attribute usability improvement was discussed at the
1/17/2020 Swift for TensorFlow open design meeting.

---

Example:

import _Differentiation
public protocol Layer {
  @differentiable
  func callAsFunction(_ input: Float) -> Float
}
public struct DummyLayer: Layer {
  public func callAsFunction(_ input: Float) -> Float { input }
}

Before:

$ swift -Xfrontend -enable-experimental-differentiable-programming toy.swift
toy.swift:6:15: error: type 'DummyLayer' does not conform to protocol 'Layer'
public struct DummyLayer: Layer {
              ^
toy.swift:7:8: note: candidate is missing attribute '@differentiable'
  func callAsFunction(_ input: Float) -> Float { input }
       ^
toy.swift:4:8: note: protocol requires function 'callAsFunction' with type '(Float) -> Float'; do you want to add a stub?
  func callAsFunction(_ input: Float) -> Float
       ^

After:

$ swift -Xfrontend -enable-experimental-differentiable-programming toy.swift
toy.swift:6:15: error: type 'DummyLayer' does not conform to protocol 'Layer'
public struct DummyLayer: Layer {
              ^
toy.swift:7:15: note: candidate is missing attribute '@differentiable'
  public func callAsFunction(_ input: Float) -> Float { input }
              ^
         @differentiable
toy.swift:4:8: note: protocol requires function 'callAsFunction' with type '(Float) -> Float'; do you want to add a stub?
  func callAsFunction(_ input: Float) -> Float
       ^

The second note protocol requires function 'callAsFunction' with type '(Float) -> Float'; do you want to add a stub? is a bit confusing. Disabling it would be ideal.

[rxwei]

LGTM

[dan-zheng]

@swift-ci Please test

[rxwei]

I'd suggest verifying this in Xcode before checking it in.

[dan-zheng]

In addition to fixit tests, verified that fixits behave as expected using c-arcmt-test and in Xcode.