[AutoDiff] Underscore the `Differentiable` protocol. (#27577)
Rename the `Differentiable` protocol to `_Differentiable`.
This matches upstreaming PR https://github.com/apple/swift/pull/27511,
which adds `protocol _Differentiable` in a new `Differentiable.swift` file.
`tensorflow` branch defines `typealias Differentiable = _Differentiable` for
usability. There should be no user-facing changes, except that some error
messages now show '_Differentiable' instead of 'Differentiable'.
