SI-7723 More finesse in setting INCONSTRUCTOR

It's a clunky flag used to determine very early on whether
we're in the self-call, super-call or early-init section.

In SI-6666 / fd6125428, the check was improved to consider nesting.
But, that caused this regression, as Function's haven't been
translated to classes yet, so our check for enclosing non-term
owners failed wrongly flagged definitins body of a anonymous function
as INCONSTRUCTOR.

With this patch, we correctly flag:

    class C extends D {
      // INCONSTRUCTOR
      () => {
        !INCONSTRUCTOR
      }
      // INCONSTRUCTOR
    }