Make $serializer generic constructor public

If a serializable class has generic type parameters, its serializer is not an object
and has a specialized constructor. This constructor was public in K1 and should
be public in K2 so it can be called from other serializable classes
(in case class is e.g., part of sealed hierarchy).

#KT-63402 Fixed

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

@@ -0,0 +1,28 @@
+// TARGET_BACKEND: JVM_IR
+
+// WITH_STDLIB
+
+// FILE: a.kt
+
+import kotlinx.serialization.*
+
+@Serializable
+sealed class Base {
+    abstract val id: Long
+}
+
+// FILE: b.kt
+
+import kotlinx.serialization.*
+import kotlinx.serialization.descriptors.*
+
+@Serializable
+class DerivedOtherFile<Value>(
+    override val id: Long,
+    val value: Value
+) : Base()
+
+fun box(): String {
+    val desc = Base.serializer().descriptor.kind
+    return if (desc == PolymorphicKind.SEALED) "OK" else desc.toString()
+}