Apparent parser sensitivity to the positioning of parametric impls

[richmckeever]

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

struct S<N: u32> {}
struct T<N: u32> {}

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

impl S<N> {
  const N_PLUS_1_VALUE = f<N>();
}

impl T<N> {
  const N_PLUS_2_VALUE = f<{S<N>::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.