fix: Closure lvalue capture bugfix

[alexvitkov]

Description

Problem*

Resolves #2120

Summary*

We used to not capture lvalues in closures properly, which was fixed in #2257, however a few codegen issue popped up (#2255, #2316) which still prevented the following example from verifying:

fn issue_2120() {
    let x1 = &mut 42;
    let set_x1 = |y| { *x1 = y; };

    assert(*x1 == 42);
    set_x1(44);
    assert(*x1 == 44);
    set_x1(*x1);
    assert(*x1 == 44);
}

Those are now both fixed, but the code above still doesn't verify, because in the previous PR I've forgotten to mark the closure environment's ast::Ident as mutable, which causes us to take a wrong branch later on in ssa_gen.

This PR fixes that and adds the example above as a test case.

Documentation

• This PR requires documentation updates when merged.

  • I will submit a noir-lang/docs PR.
  • I will request for and support Dev Rel's help in documenting this PR.

Additional Context

PR Checklist*

• I have tested the changes locally.
• I have formatted the changes with Prettier and/or cargo fmt on default settings.

[jfecher]

👍 I tested to ensure we still can't mutate immutable parameters as well