Parent-Task:

• SR-13166 Default derivative implementations for protocol requirements

Issue Description:

See relevant section from the differentiable programming manifesto: https://github.com/apple/swift/blob/master/docs/DifferentiableProgramming.md#default-derivatives-and-transposes.

The optional "default" component should appear after the optional "linear" component and before the optional "wrt:" clause.

The following ~fully elaborated @differentiable attribute should parse and pass syntax testing (but doesn't type-check).

@differentiable(linear, default, wrt: (x, y) where T: FloatingPoint)
func foo<T>(_ x: T, _ y: T) -> T {}