Suggest removing ```let``` if ```const let``` or ```let const``` is used

[Rageking8]

Given the following code:

fn main() {
    const let foo = 123;
}

The current output is:

   Compiling playground v0.0.1 (/playground)
error: expected identifier, found keyword `let`
 --> src/main.rs:2:11
  |
2 |     const let foo = 123;
  |           ^^^ expected identifier, found keyword

error: expected one of `:`, `;`, or `=`, found `foo`
 --> src/main.rs:2:15
  |
2 |     const let foo = 123;
  |               ^^^ expected one of `:`, `;`, or `=`

error: missing type for `const` item
 --> src/main.rs:2:11
  |
2 |     const let foo = 123;
  |           ^^^ help: provide a type for the item: `r#let: <type>`

error: could not compile `playground` due to 3 previous errors

Ideally the output should inform the user that let and const are mutually exclusive and suggest the user remove the let.

[obeis]

@rustbot claim