Suggest borrowing on fn argument that is impl AsRef

When encountering a move conflict, on an expression that is `!Copy` passed as an argument to an `fn` that is `impl AsRef`, suggest borrowing the expression.

```
error[E0382]: use of moved value: `bar`
  --> f204.rs:14:15
   |
12 |     let bar = Bar;
   |         --- move occurs because `bar` has type `Bar`, which does not implement the `Copy` trait
13 |     foo(bar);
   |         --- value moved here
14 |     let baa = bar;
   |               ^^^ value used here after move
   |
help: borrow the value to avoid moving it
   |
13 |     foo(&bar);
   |         +
```

Fix rust-lang#41708

core/src/borrow.rs

@@ -184,6 +184,7 @@ pub trait Borrow<Borrowed: ?Sized> {
 /// an underlying type by providing a mutable reference. See [`Borrow<T>`]
 /// for more information on borrowing as another type.
 #[stable(feature = "rust1", since = "1.0.0")]
+#[rustc_diagnostic_item = "BorrowMut"]
 pub trait BorrowMut<Borrowed: ?Sized>: Borrow<Borrowed> {
     /// Mutably borrows from an owned value.
     ///