Issue Description:

Currently, the differentiation transform (PullbackEmitter) always creates adjoint buffers for address-typed original values, regardless of whether the tangent type is loadable or address-only.

However, some address-only original types may have loadable tangent types. For tangent types that are loadable, it would be more efficient to use symbolic AdjointValues instead of adjoint buffers.

// Foo<T>: address-only
// Foo<T>.TangentVector: loadable
struct Foo<T>: Differentiable {
  var a: Float
  @noDerivative var b: T
}

We should make the "tangent value category" depend solely on whether the tangent type is loadable or address-only.

Discussion from #32540 (comment)