Fix BindOverload not being used for variables

[Snowy1803]

When overloading a variable, no error is produced, and the used variable is always set to the last. However, the constraint system is solved with all overloads, so if it is found unambiguous with the first choice, it crashes as GILGen then uses the last choice.

func consume(@unused value: Int) {}

func main() {
    let x = 42;
    let x = "Hello";
    consume(x);
}