Don't mark magic Serializable methods as unused

Fixes #KT-10635

idea/testData/inspections/unusedSymbol/function/implicitForSerialization.kt

@@ -0,0 +1,21 @@
+// FULL_JDK
+
+class Foo : java.io.Serializable {
+    private fun writeObject(s: java.io.ObjectOutputStream) {
+        s.toString()
+    }
+
+    private fun readObject(s: java.io.ObjectInputStream) {
+        s.toString()
+    }
+
+    fun writeReplace(): Any = Any()
+    fun readResolve(): Any = Any()
+
+    fun readObjectNoData() {
+    }
+}
+
+fun main(args: Array<String>) {
+    Foo()
+}