[AutoDiff] Support @differentiable class initializers.

[dan-zheng]

Support @differentiable class initializers in final classes.

• Allow @differentiable and @derivative attributes for original initializers
  in final classes. Reject original initializers in non-final classes.
• Add special logic in SILGenModule::getOrCreateCustomDerivativeThunk to
  thunk user-defined class initializer derivative functions to the expected
  derivative type for class initializers.

Also support differentiation for class-related casting instructions:
unchecked_ref_cast and upcast.

These instructions are generated when calling super.init or referencing
inherited superclass members.

Resolves SR-12151 and SR-12153.

---

Example:

class Super: Differentiable {
  var base: Float

  // Error: `Super` is non-final.
  @differentiable
  init(base: Float) {
    self.base = base
  }
}

final class Sub: Super {
  // No error: `Sub` is final.
  @differentiable
  override init(base: Float) {
    super.init(base: base)
  }
}

class.swift:5:4: error: '@differentiable' attribute cannot be declared on 'init' in a non-final class; consider making 'Super' final
  @differentiable
   ^

[dan-zheng]

@swift-ci Please test tensorflow

[dan-zheng]

@swift-ci Please test tensorflow