Missing error messages in shadowing errors

[ironcev]

All the shadowing errors (for consts, symbols, and generic parameters) are missing errors on the compiler output in the case of the same identifier. The reason is using Idents in the errors and extracting spans from them. This is exactly what we should do, but since the span is not a part of the Ident's hash, two different error messages (different spans) with the sameIdent e.g. a variable or a constant, will end up having the same hash and will be removed by the deduplication of the error messages which differentiate the messages based on their hash.

The solution is to use IdentUnique instead of Ident.

To reproduce, create a script with lib.sw like this:

library;

pub struct Struct { }

and the main.sw:

script;

mod lib;

use lib::Struct;
use lib::Struct;
use lib::Struct;

const X = 0;

fn main() -> () {
    let X = 1;
    let X = 2;

    let y = 3;

    {
        const y = 4;
    }

    {
        const y = 6;
    }
}

fn generic<T, T, T>(_x: T) { }

When compiling the script, the shadowing messages will appear only for the first error. The second one will be falsely removed by the deduplication.

Related to #4818.