[AutoDiff] Require same access level for original/derivative functions.

[dan-zheng]

Require @derivative functions and their original functions to have the same access level.
Public original functions may also have internal @usableFromInline derivatives, as a special case. Diagnose access level mismatches.

This simplifies derivative registration rules, and may enable simplification of AutoDiff symbol linkage rules.

Resolves TF-1099 and TF-1160.

---

Example:

import _Differentiation

public func publicOriginal(_ x: Float) -> Float { x }

@derivative(of: publicOriginal)
private func privativeDerivative(_ x: Float) -> (value: Float, pullback: (Float) -> Float) {
  fatalError()
}

$ swift example.swift
example.swift:5:17: error: derivative function must have same access level as original function; derivative function 'privativeDerivative' is private, but original function 'publicOriginal' is public
@derivative(of: publicOriginal)
                ^
example.swift:6:14: note: mark the derivative function as 'public' to match the original function
private func privativeDerivative(_ x: Float) -> (value: Float, pullback: (Float) -> Float) {
~~~~~~~      ^
public

---

Future direction: anonymous func declarations (SR-12003) will enable public derivative functions without exposing them as public API.

public func bar(_ x: Float) -> Float { x }

@derivative(of: bar)
public func _(_ x: Float) -> (value: Float, differential: (Float) -> Float) {
  (x, { dx in dx })
}

---

This additional restriction will break users. Fix all @derivative usage errors caught by "community CI" before merging.

Done: tensorflow/swift-apis#929

[dan-zheng]

@swift-ci Please smoke test

[rxwei]

This additional restriction will break users.

Well, this will break users by bringing possible linker errors to compile-time errors :)

[dan-zheng]

@swift-ci Please test

[swift-ci]

Build failed
Swift Test Linux Platform
Git Sha - a6bba14a2d7c6bee70e0da20c98bc65bdddd2082

[swift-ci]

Build failed
Swift Test OS X Platform
Git Sha - a6bba14a2d7c6bee70e0da20c98bc65bdddd2082