[AutoDiff upstream] Relax @differentiable for protocol witnesses.

[dan-zheng]

Previously, all witnesses of a @differentiable protocol requirement were
required to have the same attribute (or one with superset parameter indices).

However, this leads to many annotations on witnesses and is not ideal for
usability. @differentiable attributes are really only significant on
public witnesses, so that they are clearly @differentiable at a glance (in
source code, interface files, and API documentation), without looking through
protocol conformance hierarchies.

Now, only public witnesses of @differentiable protocol requirements are
required to have the same attribute (or one with superset parameter indices).
For less-visible witnesses, an implicit @differentiable attribute is created
with the same configuration as the requirement's.

Resolves TF-1117.
Upstreams #29771 from tensorflow branch.

---

Example:

public protocol Layer: Differentiable {
  associatedtype Input: Differentiable
  associatedtype Output: Differentiable
  @differentiable(wrt: (self, input))
  func callAsFunction(_ input: Input) -> Output
}

// Internal conforming type.
struct DummyInternalLayer: Layer {
  func callAsFunction(_ input: Float) -> Float {
    return input
  }
}

Before (misleading diagnostic due to TF-1014):

layer.swift:9:8: error: type 'DummyInternalLayer' does not conform to protocol 'Layer'
struct DummyInternalLayer: Layer {
       ^
layer.swift:2:18: note: protocol requires nested type 'Input'; do you want to add it?
  associatedtype Input: Differentiable
                 ^
layer.swift:3:18: note: protocol requires nested type 'Output'; do you want to add it?
  associatedtype Output: Differentiable
                 ^

After: no error.

An implicit @differentiable attribute is created for DummyInternalLayer.callAsFunction:

$ swiftc -print-ast layer.swift
...

internal struct DummyInternalLayer : Layer {
  @differentiable(wrt: (self, input))
  internal func callAsFunction(_ input: Float) -> Float
  ...
}

[dan-zheng]

@swift-ci Please smoke test

[dan-zheng]

Merging to unblock progress. #29771 was previously approved.