Using the same facility that we use to record subclasses of
sealed classes, record the subclasses of term-owned ("local")
classes.

I have changed existing callers of `children` to use `sealedChildren`
so we don't start using this new information in pattern matching
and type pattern checkability analysis.

The following commit will build on this to infer finality of local
classes in the context of outer pointer elision in the constructors
phase.

 - Check if the clazz that owns all the decls we're filtering is
effectively final once, rather than for each decl.
 - Query the full set of decls once.

Previously, only local classes declared final would be candidates
for outer pointer elision in the constructor phase.

This commit infers finality of local classes to expand the scope
of this optimization.

== Background ==

This was brought to our attention when shapeless enabled
indylambda and found that a hitherto serializable
data structure started to capture the enclosing class and hence
lost its serializability.

    class NotSerializable {
      def test = () => {
        class C; assertSerializable(new C)
      }
    }

Under `-Ydelambdafy:inline`, it used to capture the enclosing anon
function class as its outer, which, being final, didn't in turn
capture the enclosing class.

    class NotSerializable {
      def test = new anonFun$1
    }
    class anonFun$1 {
      def apply = assertSerializable(new C(this))
    }
    class ...$C(outer$: anonFun)

indylambda perturbs the enclosing structure of the function body.

    class NotSerializable {
      def anonFun$1 = {class C; assertSerializable(new C()))
      def test = lambdaMetaFactory(<<anonFun$1>>)
    }

Which leads to:

    class NotSerializable$C(outer$: NotSerializable)