Correctly handle star projections according to logic of the old FE.

#KT-54297 Fixed

Properly substitute surrogate UnitSerializer in the Companion.serializer()
function generated on classes that use polymorphic or sealed serializer
by default. (Fixes https://github.com/Kotlin/kotlinx.serialization/issues/1692)

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

@@ -0,0 +1,33 @@
+// TARGET_BACKEND: JVM_IR
+
+// WITH_STDLIB
+
+import kotlinx.serialization.*
+import kotlinx.serialization.json.*
+import kotlinx.serialization.descriptors.*
+
+interface SomeInterface
+
+@Serializable
+sealed class SealedMultiple<T1 : SomeInterface, T2 : SomeInterface>
+
+@Serializable
+abstract class AbstractMultiple<T1 : SomeInterface, T2 : SomeInterface>
+
+@Serializable
+sealed interface SealedInterfaceMultiple<T1 : SomeInterface, T2 : SomeInterface>
+
+interface InterfaceMultiple<T1 : SomeInterface, T2 : SomeInterface>
+
+@Serializable
+class ToSerialize(
+    val a: SealedMultiple<SomeInterface, SomeInterface>,
+    val b: AbstractMultiple<*, *>,
+    val c: SealedInterfaceMultiple<*, *>,
+    val d: InterfaceMultiple<*, *>
+)
+
+fun box(): String {
+    val s = ToSerialize.serializer().descriptor.elementDescriptors.joinToString { it.kind.toString() }
+    return if (s == "SEALED, OPEN, SEALED, OPEN") "OK" else s
+}