Parent-Task:

• SR-13149 @derivative and @transpose type-checking diagnostic improvements

Issue Description:

It's not always obvious what should be the type of a custom derivative function, given an original function and wrt differentiability parameters.

It would be nice for the compiler to reliably show the correct derivative type in @derivative attribute type-checking diagnostics. This enables the workflow of "declaring a dummy derivative function declaration, then changing its type to the compiler-suggested correct type":

@differentiable(where T: Differentiable)
func foo<T, U>(_ x: T, _ y: inout Float, _ z: U) {}

@derivative(of: foo)
func vjpFoo() {}

Currently, the expected derivative type isn't reliably shown:

autodiff.swift:4:2: error: '@derivative(of:)' attribute requires function to return a two-element tuple; first element must have label 'value:' and second element must have label 'pullback:' or 'differential:'
@derivative(of: foo)
 ^

But we can reliably show it if the original function declaration and differentiability parameters can be resolved.