[AutoDiff] Support @differentiable class initializers.

[dan-zheng]

Support @differentiable class initializers.

• Add special logic in SILGenModule::getOrCreateCustomDerivativeThunk to
  thunk user-defined class initializer derivative functions to the expected
  derivative type for class initializers.
• Make minor vtable SILGen changes for class initializer derivatives.

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.

[dan-zheng]

Note: I added upcast and unchecked_ref_cast differentiation support in this patch because they're needed to make this superclass initializer as @differentiable.

Otherwise, inherited initializers encounter non-differentiability errors at weird source locatiosn for unchecked_ref_cast and upcast instructions.

[dan-zheng]

Note: this added logic in TypeBase::replaceCovariantResultType is hacky. It's necessary for TypeConverter::getConstantOverrideInfo (derivative vtable entry SILGen) to work properly.

I can't think of a more robust alternative that doesn't involve ~60 lines of code dupe.

---

One alternative is to drop support for @differentiable initializers in non-final classes.

Initializers return a dynamic Self type, so perhaps non-final initializer derivatives shouldn't be supported (since derivatives would return a tuple (Self, ...) and type-checking accepts covariant Self only as a top-level method result type).

If there are no use cases for @differentiable initializers in non-final classes, I think dropping support is fine.

[rxwei]

I believe we should ban @differentiable on everything that produces a covariant result.

[dan-zheng]

Verifying whether tests pass.
@swift-ci Please test tensorflow

[rxwei]

Could you please add a test case where a convenience initializer takes an address-only parameter? For example:

final class Foo<T> {
    ...
    @differentiable
    convenience init(_ x: T) {
        self.init(...)
    }
}

[rxwei]

This is in TypeBase. getAs() would be more idiomatic.

Suggested change

	if (auto tupleType = dyn_cast<TupleType>(this)) {
	if (auto tupleType = getAs<TupleType>()) {

[dan-zheng]

Could you please add a test case where a convenience initializer takes an address-only parameter? For example:

Test added in d7b12c2. @differentiable convenience initializers seem to work just fine.

---

I believe we should ban @differentiable on everything that produces a covariant result.

That sounds good. @differentiable attribute on other class members with dynamic Self result is already diagnosed.

Not supporting @differentiable attribute in non-final classes allows us to remove the hacky logic at #29754 (comment).

[dan-zheng]

Closing in favor of #29757.

This patch supports @differentiable attribute on final and non-final class initializers, using some hacky code.

However, Swift doesn't formally support non-top-level covariant results, so #29757 allows only @differentiable class initializers in final classes.