Rollup merge of #138314 - haenoe:autodiff-inner-function, r=ZuseZ4

fix usage of `autodiff` macro with inner functions

This PR adds additional handling into the expansion step of the `std::autodiff` macro (in `compiler/rustc_builtin_macros/src/autodiff.rs`), which allows the macro to be applied to inner functions.

```rust
#![feature(autodiff)]
use std::autodiff::autodiff;

fn main() {
    #[autodiff(d_inner, Forward, Dual, DualOnly)]
    fn inner(x: f32) -> f32 {
        x * x
    }
}
```

Previously, the compiler didn't allow this due to only handling `Annotatable::Item` and `Annotatable::AssocItem` and missing the handling of `Annotatable::Stmt`. This resulted in the rather generic error

```
error: autodiff must be applied to function
 --> src/main.rs:6:5
  |
6 | /     fn inner(x: f32) -> f32 {
7 | |         x * x
8 | |     }
  | |_____^

error: could not compile `enzyme-test` (bin "enzyme-test") due to 1 previous error
```

This issue was originally reported [here](https://github.com/EnzymeAD/rust/issues/184).

Quick question: would it make sense to add a ui test to ensure there is no regression on this?
This is my first contribution, so I'm extra grateful for any piece of feedback!! :D

r? `@oli-obk`

Tracking issue for autodiff: #124509