Marked write$Self and $serializer from serialization as synthetic

plugins/kotlinx-serialization/kotlinx-serialization.backend/src/org/jetbrains/kotlinx/serialization/compiler/backend/ir/SerializableIrGenerator.kt

@@ -369,6 +369,7 @@ class SerializableIrGenerator(
         val serializerDescriptor = irClass.classSerializer(compilerContext)?.owner ?: return
         if (irClass.shouldHaveSpecificSyntheticMethods { serializerDescriptor.findPluginGeneratedMethod(SAVE) }) {
             val func = irClass.findWriteSelfMethod() ?: return
+            func.origin = SERIALIZATION_PLUGIN_ORIGIN
             generateWriteSelfMethod(func)
         }
     }

plugins/kotlinx-serialization/kotlinx-serialization.backend/src/org/jetbrains/kotlinx/serialization/compiler/backend/ir/SerializerIrGenerator.kt

@@ -623,6 +623,7 @@ open class SerializerIrGenerator(
                 else -> SerializerIrGenerator(irClass, context, metadataPlugin)
             }
             generator.generate()
+            irClass.origin = SERIALIZATION_PLUGIN_ORIGIN
             irClass.addDefaultConstructorIfAbsent(context)
             irClass.patchDeclarationParents(irClass.parent)
         }