Termination rule should also apply to instantiating specifics

[josh11b]

In this example:

interface I {
  let T:! type;
  default fn F() -> T;
}

fn I.F() -> T {
  class C {
    // `I` is complete, but `I.F`'s default
    // value is still being defined.
    impl as I where .T = i32 {}
  }
}

If the default definition of I.F is eveer used, it will lead to an infinite instantiation loop. This is because forming the specific for a given type U will trigger creation of a new class C(U), which itself has an impl of I that uses the default value of I.F.

We should apply the termination check to formation of specifics to detect this problem, since each instantiation is strictly more complex.