Add missing semicolon suggestion for global variable declarations

[Rageking8]

Given the following code: link

const x: i32 = 123

fn main() {

}

The current output is:

   Compiling playground v0.0.1 (/playground)
error: expected one of `.`, `;`, `?`, or an operator, found keyword `fn`
 --> src/main.rs:3:1
  |
1 | const x: i32 = 123
  |                   - expected one of `.`, `;`, `?`, or an operator
2 |
3 | fn main() {
  | ^^ unexpected token

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

Ideally the output should suggest putting a semicolon at the end of the declaration, since that suggestion is present if that declaration is placed in the main function.

[chenyukang]

@rustbot claim