SI-7571 Allow nesting of anonymous classes in value classes

5d9cde105e added deep prohibition of nested classes within
a value class. This has the undesirable side effect of
prohibiting partial functions literals in method bodies
of a value class.

The intention of that prohibition was to avoid problems
in code using Type Tests, such as:

    class C(val inner: A) extends AnyVal {
      class D
    }
    def foo(a: Any, other: C) = a match { case _ : other.D }

Here, the pattern usually checks that `a.$outer == other`.
But that is incongruent with the way that `other` is erased
to `A`.

However, not all nested classes could lead us into this trap.
This commit slightly relaxes the restriction to allow anonymou
classes, which can't appear in a type test.
The test shows that the translation generates working code.

Review by @gkossakowski