Function pointer signature containing `&mut T` should work in const context

[nayeemrmn]

Proposing to extract this from #57349.

The following examples shouldn't error:

const fn get_some_fn() -> fn(&mut String) {
  String::clear
}

const fn use_const_fn(_f: fn(&mut String)) {
  ()
}

const fn some_const_fn() {
  let _f: fn(&mut String) = String::clear;
}

// mutable references are not allowed in constant functions      x3

While this has been considered part of #57349 for a while, it's also been acknowledged as a bug and suggested by @RalfJung to address separately ahead of stabilizing const_mut_refs. Indeed it's surprising for users that this would count as handling a mutable reference in const contexts.

See:
#57349 (comment)
#57349 (comment)

[RalfJung]

Cc @rust-lang/wg-const-eval

Fixing this requires changing this logic to skip traversing things inside function pointers. I'm not sure what the best way to achieve that is.

[EvanMerlock]

@rustbot claim