[AutoDiff] Change Differentiable.moved(along:) to move(along:).

[dan-zheng]

Change Differentiable.moved(along:) to mutating func move(along:).
Update Differentiable derived conformances (logic and diagnostics).
Update tests.

---

Example:

struct Foo : Differentiable {
    var x, y: Float
  
    // Old: returns a new instance.
    // func moved(along direction: TangentVector) -> Foo {
    //     return Foo(x: x.moved(along: direction.x),
    //                y: y.moved(along: direction.y))
    // }
  
    // New: updates in-place.
    mutating func move(along direction: TangentVector) {
        x.move(along: direction.x)
        y.move(along: direction.y)
    }
}

This is important for upcoming Differentiable class support.
moved(along:) is problematic for classes because it returns a new instance, which is undesirable.
Instead, move(along:) updates classes in-place.

Derived conformances logic and diagnostics are updated.
One important change is: Differentiable derived conformances now generate a warning for all let stored properties, instead of only let stored properties with initial value. The reason is that synthesis of move(along:) requires all stored properties to be mutable.

Example:

struct Example: Differentiable {
    // These stored properties are fine.
    var x: Float
    var y = Float(1)

    // Warnings are emitted for these stored properties.
    // Synthesis (`TangentVector`, `move(along:)`, etc) ignores these properties.
    // They are implicitly marked with `@noDerivative`.
    let bad1: Float
    let bad2: Int

    // Example synthesis.
    // Intentionally ignores `AllDifferentiableVariables` because it will be removed soon.
    //
    // struct TangentVector: Differentiable, AdditiveArithmetic, VectorProtocol {
    //     var x: Float.AllDifferentiable
    //     var y: Float.AllDifferentiable
    // }
    //
    // mutating func move(along direction: TangentVector) {
    //     x.move(along: direction.x)
    //     y.move(along: direction.y)
    // }
}

example.swift:9:9: warning: synthesis of the 'Differentiable.move(along:)' requirement for 'Example' requires all stored properties to be mutable; use 'var' instead, or add an explicit '@noDerivative' attribute, or conform 'Example' to 'AdditiveArithmetic'
    let bad1: Float
        ^
    @noDerivative
example.swift:10:9: warning: stored property 'bad2' has no derivative because it does not conform to 'Differentiable'; add an explicit '@noDerivative' attribute, or conform 'Example' to 'AdditiveArithmetic'
    let bad2: Int
        ^
    @noDerivative

[dan-zheng]

@swift-ci Please clean test tensorflow