Lint on fn pointers comparisons in external macros

[Urgau]

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).

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 #134527