Display better snippet for invalid char literal

Display better snippet for invalid char literal

Given this code:

    fn main() {
        let _ = 'abcd';
    }

The compiler would give a message like:

    error: character literal may only contain one codepoint: ';
    let _ = 'abcd';
                 ^~

With this change, the message now displays:

    error: character literal may only contain one codepoint: 'abcd'
    let _ = 'abcd'
            ^~~~~~

Fixes #30033