Apparent parser sensitivity to the positioning of parametric impls

Using this modified code example from a test in a pending type_system_v2 change:

```
struct S {}
struct T {}

fn f() -> u32 { N + 1 }

impl S {
const N_PLUS_1_VALUE = f();
}

impl T {
const N_PLUS_2_VALUE = f<{S::N_PLUS_1_VALUE}>();
}
```

When parsing the S impl, the parser appears to deal out a NameRef to N which points to the binding NameDef in T. However, if you move the S impl to immediately after the counterpart struct, it does the right thing.