Rework and fix associated const visibility for impl items

[tritao]

Description

This PR fixes associated constant visiblity checking for impl items.

The behaviour is that consts declared in traits are always considered public, whereas if they are declared anywhere else they are private unless they have the pub modifier.

library;

pub struct I8 {
    underlying: u8,
}

impl I8 {
    const MIN: Self = Self {
        underlying: u8::min(),
    };
}

trait Foo {
    fn foo();
}

impl Foo for I8 {
    fn foo() {
        let x = Self::MIN;
        log(x);
    }
}

Fixes #6772.

Checklist

• I have linked to any relevant issues.
• I have commented my code, particularly in hard-to-understand areas.
• I have updated the documentation where relevant (API docs, the reference, and the Sway book).
  • If my change requires substantial documentation changes, I have requested support from the DevRel team
• I have added tests that prove my fix is effective or that my feature works.
• I have added (or requested a maintainer to add) the necessary Breaking* or New Feature labels where relevant.
• I have done my best to ensure that my PR adheres to the Fuel Labs Code Review Standards.
• I have requested a review from the relevant team or maintainers.

[codspeed-hq]

CodSpeed Performance Report

Merging #6785 will not alter performance

_{Comparing tritao:associated-const-visibility (11a6a77) with master (7206479)}

Summary

✅ 22 untouched benchmarks

[jjcnn]

I'm still confused by what visibility_level means.

It also took me a while to figure out what problem this PR is supposed to solve in the first place. The PR deccription just links to an issue, but that issue only consists of an example program with no description of what the compiler does incorrectly for that program.