Recover from &dyn mut ... parse errors

[FabianWolff]

Consider this example:

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

[rust-highfive]

r? @petrochenkov

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[petrochenkov]

@bors r+

[bors]

📌 Commit c692896 has been approved by petrochenkov