Show a note when closure field is called as method

Closes https://github.com/rust-lang/rust/issues/18343
I think this would actually be clearer as a match expression.

```rust
let is_field = match ty::get(rcvr_ty).sty {
    ty_struct(did, _) =>
        ty::lookup_struct_fields(cx, did)
            .iter()
            .any(|f| f.name.user_string(cx) == method_ustring),
    _ => false
}