K2 plugins: create companions with same expect/actual as their owners

#KT-59299 Fixed

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

@@ -0,0 +1,25 @@
+// LANGUAGE: +MultiPlatformProjects
+// TARGET_BACKEND: JVM_IR
+// IGNORE_BACKEND_K1: JVM_IR
+// IGNORE REASON: multimodule MPP tests are not supported for K1
+// WITH_STDLIB
+
+// MODULE: m1-common
+// FILE: common.kt
+
+import kotlinx.serialization.*
+
+@Serializable
+expect sealed class Some
+
+fun s(): KSerializer<Some> = Some.Companion.serializer()
+
+// MODULE: m2-jvm()()(m1-common)
+// FILE: jvm.kt
+
+import kotlinx.serialization.*
+
+@Serializable
+actual sealed class Some
+
+fun box(): String = if (s().descriptor.toString() == Some.serializer().descriptor.toString()) "OK" else "fail"