SI-8708 Fix pickling of LOCAL_CHILD child of sealed classes

When a sealed class or trait has local children, they are not pickled
in as part of the children of the symbol (introduced in 12a2b3b to fix
Aladdin bug 1055). Instead the compiler adds a single child class
named LOCAL_CHILD. The parents of its ClassInfoType were wrong: the
first parent should be a class. For sealed traits, we were using the
trait itself.

Also, the LOCAL_CHILD dummy class was entered as a member of its
enclosing class, which is wrong: it represents a local (non-member)
class, and it's a synthetic dummy anyway.
I think the two answer different questions. `isLocalToPickle` decides wether a symbol is declared within the class that's currently being pickled. For example, `class C[T] { val x: Int }`, the symbols for `x` and `T` are both local to pickle.

`shouldEnterInOwnerScope` on the other hand answers the question if the unpickled symbol should be entered in scope. This is true for `x`, but not for `T`.