Rollup merge of #115363 - kpreid:suggest-private, r=compiler-errors

Don't suggest adding parentheses to call an inaccessible method.

Previously, code of this form would emit E0615 (attempt to use a method as a field), thus emphasizing the existence of private methods that the programmer probably does not care about. Now it ignores their existence instead, producing error E0609 (no field). The motivating example is:

```rust
let x = std::rc::Rc::new(());
x.inner;
```

which would previously mention the private method `Rc::inner()`, even though `Rc<T>` intentionally has no public methods so that it can be a transparent smart pointer for any `T`.

```rust
error[E0615]: attempted to take value of method `inner` on type `Rc<()>`
 --> src/main.rs:3:3
  |
3 | x.inner;
  |   ^^^^^ method, not a field
  |
help: use parentheses to call the method
  |
3 | x.inner();
  |        ++
  ```

  With this change, it emits E0609 and no suggestion.

Author: matthiaskrgr