[KxSerialization] Fix "SyntheticAccessorLowering should not attempt to modify other files"

Added additional check for generated serializer to use cached child serializers

Fixes #KT-56990

Merge-request: KT-MR-9057
Merged-by: Sergey Shanshin <Sergey.Shanshin@jetbrains.com>

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

@@ -83,8 +83,9 @@ open class SerializerIrGenerator(
         private set
 
     // child serializers cached if serializable class is internal
-    private val cachedChildSerializersProperty =
+    private val cachedChildSerializersProperty = if (isGeneratedSerializer)
         serializableIrClass.companionObject()?.properties?.singleOrNull { it.name == CACHED_CHILD_SERIALIZERS_PROPERTY_NAME }
+    else null
 
     // non-object serializers which can be cached
     private val cacheableChildSerializers =