Support sealed class inheritors in the same file

#KT-11573 Fixed

compiler/testData/diagnostics/tests/sealed/DerivedTopLevel.kt

@@ -0,0 +1,10 @@
+sealed class Base
+
+class Derived: Base() {
+    class Derived2: <!INVISIBLE_MEMBER, SEALED_SUPERTYPE!>Base<!>()
+}
+
+fun test() {
+    class Local: <!INVISIBLE_MEMBER, SEALED_SUPERTYPE!>Base<!>()
+}
+