Record field is corrupted when passing from fn(x<int>) to fn(x<T>)

The following program fails:

```
type x<T> = { a: T, b: bool, c: bool };

fn main() {
    let x: x<int> = { a: 3, b: false, c: true };
    foo(x);
}

fn foo(x: x<int>) {
    assert x.c; // passes
    bar(x)
}

fn bar<T>(x: x<T>) {
    assert x.c; // fails
}
```

This is a minimal test case based on a failure I found when trying to modify code called by `std::test::run_tests_console`.
