Use correct check in FirSerializationPluginClassChecker to determine if the given type is a enum class.

Otherwise, findTypeSerializerOrContextUnchecked works incorrectly when analyzing code
compiled with Kotlin 1.7.20 and serialization 1.4.1.

#KT-57704 Fixed

plugins/kotlinx-serialization/kotlinx-serialization.k2/src/org/jetbrains/kotlinx/serialization/compiler/fir/checkers/FirSerializationPluginClassChecker.kt

@@ -536,7 +536,7 @@ object FirSerializationPluginClassChecker : FirClassChecker() {
             }
             checkTypeArguments(type, source, reporter)
         } else {
-            if (!type.isEnum) {
+            if (type.toRegularClassSymbol(session)?.isEnumClass != true) {
                 // enums are always serializable
                 reporter.reportOn(source, FirSerializationErrors.SERIALIZER_NOT_FOUND, type)
             }