In `Differentiable` derived conformances, `let` properties are currently treated as if they had `@noDerivative` and excluded from the derived `Differentiable` conformance implementation. This is limiting to properties that have a non-mutating `move(along:)` (e.g. class properties), which can be mathematically treated as differentiable variables.
This patch changes the derived conformances behavior such that `let` properties will be included as differentiable variables if they have a non-mutating `move(along:)`. This unblocks the following code:
```swift
final class Foo: Differentiable {
let x: ClassStuff // Class type with a non-mutating 'move(along:)'
// Synthesized code:
// struct TangentVector {
// var x: ClassStuff.TangentVector
// }
// ...
// func move(along direction: TangentVector) {
// x.move(along: direction.x)
// }
}
```
Resolves SR-13474 (rdar://67982207).
Fix pullback generation for `unchecked_ref_cast` and `upcast` instructions
for class types with an address tangent value category.
Upstream end-to-end class differentiation tests.
