[FIR] Don't create generated nested classifier scope for non-source classes

^KT-57626 Fixed

plugins/kotlinx-serialization/testData/boxIr/serializableFromAnotherModule.kt

@@ -0,0 +1,25 @@
+// TARGET_BACKEND: JVM_IR
+// WITH_STDLIB
+// ISSUE: KT-57626
+
+// MODULE: lib
+import kotlinx.serialization.*
+
+@Serializable
+data class A(val s: String = "")
+
+// MODULE: main(lib)
+import kotlinx.serialization.*
+import kotlinx.serialization.json.*
+
+@Serializable
+data class B(val a: A? = null)
+
+fun box(): String {
+    val expectedB = B(A("OK"))
+    val json = Json.encodeToString(B.serializer(), expectedB)
+    if (json != """{"a":{"s":"OK"}}""") return "Fail: $json"
+    val actualB = Json.decodeFromString(B.serializer(), json)
+    if (expectedB != actualB) return "Fail: $actualB"
+    return "OK"
+}