Support @Serializable(ContextualSerializer) on class

It was fixed some time ago (https://github.com/Kotlin/kotlinx.serialization/issues/957)
but for some reason only for old backend.

Fixes https://github.com/Kotlin/kotlinx.serialization/issues/1776

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

@@ -403,6 +403,7 @@ abstract class BaseIrGenerator(private val currentClass: IrClass, final override
                     // new signature of context serializer
                     args = args + mutableListOf<IrExpression>().apply {
                         val fallbackDefaultSerializer = findTypeSerializer(pluginContext, kType)
+                            .takeIf { it?.owner?.classId != contextSerializerId }
                         add(instantiate(fallbackDefaultSerializer, kType) ?: irNull())
                         add(
                             createArrayOfExpression(