update `#[derive(Eq)]` to not need `assert_receiver_is_total_eq`

#[derive(Eq)] currently uses the hidden method Eq::assert_receiver_is_total_eq to emit checks that every field of the type being derived on also implements Eq. It would be nice to be able to get rid of it entirely, which requires changing this derive macro to no longer place its checks in this method.

Some implementation notes from https://github.com/rust-lang/rust/pull/149978#issuecomment-3781908738:

For this struct:

struct Point {
    x: u32,
    y: u32,
}

Instead of generating this:

#[automatically_derived]
impl ::core::cmp::Eq for Point {
    #[inline]
    #[doc(hidden)]
    #[coverage(off)]
    #[allow(internal_eq_trait_method_impls)]
    fn assert_receiver_is_total_eq(&self) -> () {
        let _: ::core::cmp::AssertParamIsEq<u32>;
    }
}

You would generate this:

#[automatically_derived]
impl ::core::cmp::Eq for Point {}
const _: () = {
    #[doc(hidden)]
    #[coverage(off)]
    fn assert_receiver_is_total_eq() {
        let _: ::core::cmp::AssertParamIsEq<u32>;
    }
};

One problem is that the const should probably also have #[automatically_derived] put on it, but that's currently not permitted.

https://github.com/rust-lang/rust/pull/149978#discussion_r2795421972 contains some more notes on this.

Tracking issue: https://github.com/rust-lang/rust/issues/152336