Fixed replacing serializer origin for generated classes

Do not use plugin origin for user-written classes that have @Serializer annotation for some reason. Plugin origin makes them synthetic which is incorrect because they're available in source code. Example of such class: @Serializer(JsonElement::class) class JsonElementSerializer

Merge-request: KT-MR-7627
Merged-by: Sergey Shanshin <Sergey.Shanshin@jetbrains.com>

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

@@ -624,7 +624,10 @@ open class SerializerIrGenerator(
                 else -> SerializerIrGenerator(irClass, context, metadataPlugin)
             }
             generator.generate()
-            irClass.origin = SERIALIZATION_PLUGIN_ORIGIN
+            if (irClass.isFromPlugin(context.afterK2)) {
+                // replace origin only for plugin generated serializers
+                irClass.origin = SERIALIZATION_PLUGIN_ORIGIN
+            }
             irClass.addDefaultConstructorIfAbsent(context)
             irClass.patchDeclarationParents(irClass.parent)
         }