Recover from mutable variable declaration where ```mut``` is placed before ```let```

[Rageking8]

Given the following code:

fn main() {
    mut let x = 123;
}

The current output is:

   Compiling playground v0.0.1 (/playground)
error: expected expression, found keyword `mut`
 --> src/main.rs:2:5
  |
2 |     mut let x = 123;
  |     ^^^ expected expression

error: could not compile `playground` due to previous error

Ideally the output should suggest to move the mut to behind the let, since the current error message does not identify the problem accurately.

[obeis]

@rustbot claim