Fixes issue #29124.

If method is called on a function type a note is generated to suggest
that the developer may have forgotten to call it.

e.g.

fn main() {
    let mut guess = String::new();
    std::io::stdin.read_line(&mut guess);
}

will generate the note:

note: called method on function type. did you mean `std::io::stdin().read_line(..)`?

Split `fileline_note` into a `file_line note` and `span_suggestion` as per
@Manishearth's suggestions.

Change nested `match`es to `if let`s.

PR: #32053