Lint on fn pointers comparisons in external macros
This PR extends the recently stabilized `unpredictable_function_pointer_comparisons` lint ~~to also lint on `Option<{function pointer}>` and~~ as well as linting in external macros (as to catch `assert_eq!` and others).
```rust
assert_eq!(Some::<FnPtr>(func), Some(func as unsafe extern "C" fn()));
//~^ WARN function pointer comparisons
#[derive(PartialEq, Eq)]
struct A {
f: fn(),
//~^ WARN function pointer comparisons
}
```
Fixes https://github.com/rust-lang/rust/issues/134527
ae7615039f