[AutoDiff] Disallow differentiation of opaque-result-typed functions.

[dan-zheng]

Reject @differentiable and @derivative attribute for original functions with opaque result types.

It is not possible to support derivative registration nor the differentiation transform for such functions.

There's no way to express the proper derivative type for functions returning some Differentiable:

func foo(_ x: Float) -> some Differentiable { x }

@derivative(of: foo)
// The expected JVP/VJP type isn't currently expressible:
func vjpFoo(_ x: Float) -> <T: Differentiable> (value: T, pullback: (T.TangentVector) -> TangentVector) { ... }

Aside from derivative registration, there's no principled way to "unpack" values of type (some Differentiable).TangentVector to get the underlying value, which would be necessary in the differentiation transform.

sil private [ossa] @AD__foo__pullback_src_0_wrt_0 : $@convention(thin) (@in_guaranteed @_opaqueReturnTypeOf("foo", 0) 🦸.TangentVector, @owned _AD__foo_bb0__PB__src_0_wrt_0) -> Float {
bb0(%0 : $*@_opaqueReturnTypeOf("foo", 0) 🦸.TangentVector, %1 : $_AD__foo_bb0__PB__src_0_wrt_0):
  // How to unpack `$*@_opaqueReturnTypeOf("foo", 0) 🦸.TangentVector` to the actual
  // underlying non-opaque type (e.g. `$*Float`)?
  //
  // We could perform some unsafe type coercion (`unchecked_addr_cast`), not sure about the full implications.
}

Resolves SR-12656: differentiation transform crash.

[dan-zheng]

@swift-ci Please smoke test

[dan-zheng]

@swift-ci Please smoke test