Proper handling of intrinsics with `#[rustc_intrinsic_must_be_overridden]`

[celinval]

rust-lang/rust#120675 introduced a new mechanism to declare intrinsics which will potentially replace the rust-intrinsic ABI. The new mechanism introduces a dummy body and mark the intrinsic with #[rustc_intrinsic_must_be_overridden].

I believe we can maintain the same interface we have so far, and we can change the has_body check for instances. For the case of an intrinsic, we return that a body is not available if rustc_intrinsic_must_be_overridden annotation is present.

@oli-obk, would that make sense?