…ef, r=estebank

Rework ptr-to-ref conversion suggestion for method calls

If we have a value `z` of type `*const u8` and try to call `z.to_string()`, the upstream compiler will show you a note suggesting to call `<*const u8>::as_ref` first.

This PR extends that:
- The note will only be shown when the method would exist on the corresponding reference type
- It can now suggest any of `<*const u8>::as_ref`, `<*mut u8>::as_ref` and `<*mut u8>::as_mut`, depending on what the method needs.

I didn't introduce a `help` message because that's not a good idea with `unsafe` functions (and you'd also need to unwrap the `Option<&_>` somehow).
People should check the safety requirements.

For the simplest case
```rust
fn main() {
    let x = 8u8;
    let z: *const u8 = &x;
    // issue rust-lang#21596
    println!("{}", z.to_string()); //~ ERROR E0599
}
```
the output changes like this:
```diff
 error[E0599]: `*const u8` doesn't implement `std::fmt::Display`
   --> $DIR/suggest-convert-ptr-to-ref.rs:5:22
    |
 LL |     println!("{}", z.to_string());
    |                      ^^^^^^^^^ `*const u8` cannot be formatted with the default formatter
    |
-   = note: try using `<*const T>::as_ref()` to get a reference to the type behind the pointer: https://doc.rust-lang.org/std/primitive.pointer.html#method.as_ref
-   = note: using `<*const T>::as_ref()` on a pointer which is unaligned or points to invalid or uninitialized memory is undefined behavior
+note: the method `to_string` exists on the type `&u8`
+  --> $SRC_DIR/alloc/src/string.rs:LL:COL
+   = note: try using the unsafe method `<*const T>::as_ref` to get an optional reference to the value behind the pointer: https://doc.rust-lang.org/std/primitive.pointer.html#method.as_ref
    = note: the following trait bounds were not satisfied:
            `*const u8: std::fmt::Display`
            which is required by `*const u8: ToString`
```

I removed the separate note about the safety requirements because it was incomplete and the linked doc page already has the information you need.

Fixes rust-lang#83695, but that's more of a side effect. The upstream compiler already suggests the right method name here.