improper_ctypes diagnostic should advise to avoid improper C types because their ABI is unstable

[pnkfelix]

Spawned off of this zulip thread.

Given the following code:

extern {
    fn __enzyme_autodiff(_: usize, a: &[f32], d_a: &mut [f32]);
}

The current output is:

warning: `extern` block uses type `[f32]`, which is not FFI-safe
 --> src/lib.rs:2:39
  |
2 |     fn __enzyme_autodiff(_: usize, a: &[f32], d_a: &mut [f32]);
  |                                       ^^^^^^ not FFI-safe
  |
  = note: `#[warn(improper_ctypes)]` on by default
  = help: consider using a raw pointer instead
  = note: slices have no C equivalent

warning: `extern` block uses type `[f32]`, which is not FFI-safe
 --> src/lib.rs:2:52
  |
2 |     fn __enzyme_autodiff(_: usize, a: &[f32], d_a: &mut [f32]);
  |                                                    ^^^^^^^^^^ not FFI-safe
  |
  = help: consider using a raw pointer instead
  = note: slices have no C equivalent

warning: 2 warnings emitted

Ideally the output should look like:

warning: `extern` block uses type `[f32]`, which is not FFI-safe
 --> src/lib.rs:2:39
  |
2 |     fn __enzyme_autodiff(_: usize, a: &[f32], d_a: &mut [f32]);
  |                                       ^^^^^^ not FFI-safe
  |
  = note: `#[warn(improper_ctypes)]` on by default
  = help: consider using a raw pointer instead
  = note: slices have no C equivalent, and their ABI is unstable

warning: `extern` block uses type `[f32]`, which is not FFI-safe
 --> src/lib.rs:2:52
  |
2 |     fn __enzyme_autodiff(_: usize, a: &[f32], d_a: &mut [f32]);
  |                                                    ^^^^^^^^^^ not FFI-safe
  |
  = help: consider using a raw pointer instead
  = note: slices have no C equivalent, and their ABI is unstable

warning: 2 warnings emitted

Why: Currently, people might reverse-engineer the current ABI, and use a Rust type in an extern "C" block despite the warning. Reminding the reader that the ABI is unstable will help discourage that practice.