Type variables do not appear in TModel defs/uses

[toinehartman]

Describe the bug

Type variable do not appear in TModel defs nor uses.

To Reproduce

void typeVar() {
    println("=== Type variables ===");
    tm = typeCheck("&T f(&T t) = t;");

    defs = {d.defined | Define d:<_, _, _, typeVarId(), _, _> <- tm.defines};

    // No definitions, nor uses
    print("Defs - of role `typeVarId()`: ");
    iprintln(defs);

    uses = tm.useDef o tm.defines<defined, idRole>;
    print("Uses - of role `typeVarId()`: ");
    iprintln(uses);
    println("======================\n");
}

Expected behavior
Defs and uses to exist.