Show a note when closure field is called as method

Closes https://github.com/rust-lang/rust/issues/18343
I think it'd be better to use a help diagnostic for this:

```rust
self.tcx.sess.span_help(span,
    format!("write `(s.{0})(...)` if you meant to call the \
                  function stored in the `{0}` field.", method_ustring).as_slice());
```

after the fcx.type_error_message() call.