Write/load subclasses of sealed classes in metadata

Note that now DeserializedClassDescriptor.getSealedSubclasses works a lot
faster than before, for all newly compiled sealed classes except empty ones. It
may be optimized further if we look at the metadata version of the file the
class was loaded from, however it's not easy currently because
DeserializedClassDescriptor is declared in common (non-JVM) code and has no
direct access to the binary version information.

This will also allow to add a reflection API to get subclasses of a sealed
class

 #KT-12795 Fixed

compiler/serialization/src/org/jetbrains/kotlin/serialization/DescriptorSerializer.kt

@@ -28,6 +28,7 @@ import org.jetbrains.kotlin.resolve.DescriptorUtils.isEnumEntry
 import org.jetbrains.kotlin.resolve.MemberComparator
 import org.jetbrains.kotlin.resolve.constants.NullValue
 import org.jetbrains.kotlin.resolve.descriptorUtil.builtIns
+import org.jetbrains.kotlin.resolve.descriptorUtil.classId
 import org.jetbrains.kotlin.types.*
 import org.jetbrains.kotlin.utils.Interner
 import java.io.ByteArrayOutputStream
@@ -116,6 +117,10 @@ class DescriptorSerializer private constructor(
             }
         }
 
+        for (sealedSubclass in classDescriptor.sealedSubclasses) {
+            builder.addSealedSubclassFqName(getClassifierId(sealedSubclass))
+        }
+
         val companionObjectDescriptor = classDescriptor.companionObjectDescriptor
         if (companionObjectDescriptor != null) {
             builder.companionObjectName = getSimpleNameIndex(companionObjectDescriptor.name)