[KxSerialization] Fix SERIALIZER_NOT_FOUND diagnostic for enums

If enum class from dependencies is used, which was compiled by a new version of the plugin, which uses a factory and does not create a nested `$serializer` class, then an `SERIALIZER_NOT_FOUND` diagnostic error is thrown for such an enumeration.
This happens if the old serialization runtime is used for the current project - in this case, the serializer is taken from the nested class `$serializer`. Since it is missing, the diagnostics does not work correctly.
It is acceptable for enumerations to ignore this error, because we know that enumerations are always serializable.

Merge-request: KT-MR-8818
Merged-by: Sergey Shanshin <Sergey.Shanshin@jetbrains.com>

plugins/kotlinx-serialization/kotlinx-serialization.k1/src/org/jetbrains/kotlinx/serialization/compiler/diagnostic/SerializationPluginDeclarationChecker.kt

@@ -25,6 +25,7 @@ import org.jetbrains.kotlin.resolve.descriptorUtil.*
 import org.jetbrains.kotlin.resolve.lazy.descriptors.LazyAnnotationDescriptor
 import org.jetbrains.kotlin.resolve.source.getPsi
 import org.jetbrains.kotlin.types.KotlinType
+import org.jetbrains.kotlin.types.typeUtil.isEnum
 import org.jetbrains.kotlin.types.typeUtil.supertypes
 import org.jetbrains.kotlin.util.slicedMap.Slices
 import org.jetbrains.kotlin.util.slicedMap.WritableSlice
@@ -501,7 +502,10 @@ open class SerializationPluginDeclarationChecker : DeclarationChecker {
             checkSerializerNullability(type, serializer.defaultType, element, trace, fallbackElement)
             checkTypeArguments(module, type, element, trace, fallbackElement)
         } else {
-            trace.report(SerializationErrors.SERIALIZER_NOT_FOUND.on(element ?: fallbackElement, type))
+            if (!type.isEnum()) {
+                // enums are always serializable
+                trace.report(SerializationErrors.SERIALIZER_NOT_FOUND.on(element ?: fallbackElement, type))
+            }
         }
     }