Issue Description:

The @differentiable attribute has been temporarily removed from FloatingPoint.addingProduct(:🙂 in #21554

This is because the attribute requires differentiation with respect to self, but FloatingPoint does not yet conform to Differentiable:

public protocol FloatingPoint : ... {
  @differentiable(wrt: (self, .0, .1), adjoint: _adjointAddingProduct)
  public func addingProduct(_ lhs: Self, _ rhs: Self) -> Self {
    ...
  }
}

@marcrasi attempted to conform FloatingPoint to Differentiable but ran into stdlib compilation failures described here.

Differentiation should be re-enabled for FloatingPoint.addingProduct(:🙂 when possible (after we can conform FloatingPoint to Differentiable). Note that the @differentiable(wrt: ...) parameter indices syntax may change after SR-9488 is resolved.