[FE] Collect inheritors of sealed classes from new places in computeSealedSubclasses

#KT-13495

core/descriptors/src/org/jetbrains/kotlin/resolve/DescriptorUtils.kt

@@ -377,7 +377,7 @@ fun ClassifierDescriptor.getAllSuperClassifiers(): Sequence<ClassifierDescriptor
 // Note this is a generic and slow implementation which would work almost for any subclass of ClassDescriptor.
 // Please avoid using it in new code.
 // TODO: do something more clever instead at call sites of this function
-fun computeSealedSubclasses(sealedClass: ClassDescriptor): Collection<ClassDescriptor> {
+fun computeSealedSubclasses(sealedClass: ClassDescriptor, freedomForSealedInterfacesSupported: Boolean): Collection<ClassDescriptor> {
     if (sealedClass.modality != Modality.SEALED) return emptyList()
 
     val result = linkedSetOf<ClassDescriptor>()
@@ -396,9 +396,16 @@ fun computeSealedSubclasses(sealedClass: ClassDescriptor): Collection<ClassDescr
         }
     }
 
-    val container = sealedClass.containingDeclaration
+    val container = if (!freedomForSealedInterfacesSupported) {
+        sealedClass.containingDeclaration
+    } else {
+        sealedClass.parents.firstOrNull { it is PackageFragmentDescriptor }
+    }
     if (container is PackageFragmentDescriptor) {
-        collectSubclasses(container.getMemberScope(), collectNested = false)
+        collectSubclasses(
+            container.getMemberScope(),
+            collectNested = freedomForSealedInterfacesSupported
+        )
     }
     collectSubclasses(sealedClass.unsubstitutedInnerClassesScope, collectNested = true)
     return result

core/deserialization/src/org/jetbrains/kotlin/serialization/deserialization/descriptors/DeserializedClassDescriptor.kt

@@ -165,7 +165,7 @@ class DeserializedClassDescriptor(
         }
 
         // This is needed because classes compiled with Kotlin 1.0 did not contain the sealed_subclass_fq_name field
-        return computeSealedSubclasses(this)
+        return computeSealedSubclasses(this, freedomForSealedInterfacesSupported = false)
     }
 
     override fun getSealedSubclasses() = sealedSubclasses()