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/testData/boxIr/contextualByDefault.kt

@@ -0,0 +1,16 @@
+// TARGET_BACKEND: JVM_IR
+
+// WITH_STDLIB
+
+import kotlinx.serialization.*
+
+@Serializable(with = ContextualSerializer::class)
+class Ref(
+    val id: String,
+)
+
+fun box(): String {
+    val kind = Ref.serializer().descriptor.kind.toString()
+    if (kind != "CONTEXTUAL") return kind
+    return "OK"
+}