SI-6666 Restrict hidden `this` access in self/super calls.

Detect when classes (user authored or compiler generated)
local to a self or super constructor argument would require
premature access to the in-construction instance.

The same restriction applies for classes and objects; for objects,
the premature access would result in a null via MODULE$ field.

A residual error has been lodged as SI-6997.

I'd like to remove calls to `Symbol#outerClass` (which relies on
the flaky flag INCONSTRUCTOR, see my comments in the JIRA issue
for more discussion) from `LambdaLift` and `ExplicitOuter`, and
instead use the stack of active self/super calls to know when to
skip an enclosing class. That will obviate that flag.
Merge pull request #1938 from retronym/ticket/6666

SI-6666 Restrict hidden `this` access in self/super calls.