Error messages for type variables use the wrong name

[Marwes]

Due to the convert pass renaming all variables in each builtin statement, any errors referring to these variables will not match what the source says. We should change the convert pass to ensure that they match

From #4788 (the error refers to B, which does not exist in the signature)

#[test]
fn variables_used_in_label_position_must_have_label_kind() {
    test_error_msg! {
        config: AnalyzerConfig{
            features: vec![Feature::LabelPolymorphism],
            ..AnalyzerConfig::default()
        },
        src: r#"
            builtin abc: (record: { A with T: time }, ?timeColumn: T = "_time") => int
        "#,
        expect: expect![[r#"
            error: variable B lacks the Label constraint
              ┌─ main:2:13
              │
            2 │             builtin abc: (record: { A with T: time }, ?timeColumn: T = "_time") => int
              │             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

        "#]],
    }
}