Support `#[diagnostic::on_unimplemented]` on methods

The following code

struct SetFoo;
struct SetBar;

struct S<Foo, Bar>(Foo, Bar);
impl S<SetFoo, ()> {
    #[diagnostic::on_unimplemented(message= "builder with missing mandatory fields", label="you must set every mandatory field")]
    fn build() {}
}

fn main() {
    S::<(), SetBar>::build();
}

should let you customize the output of E0599. Currently, it is

error[E0599]: no function or associated item named `build` found for struct `S<(), SetBar>` in the current scope
  --> src/main.rs:11:22
   |
 4 | struct S<Foo, Bar>(Foo, Bar);
   | ------------------ function or associated item `build` not found for this struct
...
11 |     S::<(), SetBar>::build();
   |                      ^^^^^ function or associated item not found in `S<(), SetBar>`
   |
   = note: the function or associated item was found for
           - `S<SetFoo, ()>`

but it should allow you to output

error[E0599]: builder with missing mandatory fields
  --> src/main.rs:11:22
   |
 4 | struct S<Foo, Bar>(Foo, Bar);
   | ------------------ function or associated item `build` not found for this struct
...
11 |     S::<(), SetBar>::build();
   |                      ^^^^^ you must set every mandatory field
   |
  = note: function or associated item not found in `S<(), SetBar>`
   = note: the function or associated item was found for
           - `S<SetFoo, ()>`

Reproduce the provided Rust example and inspect how E0599 is produced for an associated item that exists only on another instantiation. Trace how #[diagnostic::on_unimplemented] is handled for traits, then determine the corresponding method behavior; done means the example emits the requested custom message, label, and remaining notes.