Rollup merge of #86812 - FabianWolff:recover-dyn-mut, r=petrochenkov

Recover from `&dyn mut ...` parse errors

Consider this example:
```rust
fn main() {
    let r: &dyn mut Trait;
}
```
This currently leads to:
```
error: expected one of `!`, `(`, `;`, `=`, `?`, `for`, lifetime, or path, found keyword `mut`
 --> src/main.rs:2:17
  |
2 |     let r: &dyn mut Trait;
  |                 ^^^ expected one of 8 possible tokens

error: aborting due to previous error
```
However, especially for beginners, I think it is easy to get `&dyn mut` and `&mut dyn` confused. With my changes, I get a help message, and the parser even recovers:
```
error: `mut` must precede `dyn`
 --> test.rs:2:12
  |
2 |     let r: &dyn mut Trait;
  |            ^^^^^^^^ help: place `mut` before `dyn`: `&mut dyn`

error[E0405]: cannot find trait `Trait` in this scope
 --> test.rs:2:21
  |
2 |     let r: &dyn mut Trait;
  |                     ^^^^^ not found in this scope

error: aborting due to 2 previous errors
```

0 files changed, 0 insertions, 0 deletions