Check for name when searching Companion.serializer() function to generate

Name check was forgotten during refactoring, so any user function with
matching signature could be overwritten by the plugin, while correct
function would not have body.

#KT-55682 Fixed

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

@@ -0,0 +1,37 @@
+// TARGET_BACKEND: JVM_IR
+
+// WITH_STDLIB
+
+import kotlinx.serialization.*
+import kotlinx.serialization.descriptors.*
+import kotlinx.serialization.builtins.*
+
+
+@Serializable
+data class Main(val fields: MainFields) {
+    companion object  {
+        fun fieldsSerializer(): KSerializer<MainFields> = MainFields.serializer()
+    }
+}
+
+@Serializable
+data class MainFields(val firstName: String?)
+
+@Serializable
+data class Box<T>(val boxed: T) {
+    companion object {
+        fun <T> serializerLike(tSer: KSerializer<T>): KSerializer<List<Box<T>>> = ListSerializer(serializer(tSer))
+    }
+}
+
+
+fun box(): String {
+    if (Main.fieldsSerializer().descriptor.toString() != "MainFields(firstName: kotlin.String?)") return "Error1"
+    if (MainFields.serializer().descriptor.toString() != "MainFields(firstName: kotlin.String?)") return "Error2"
+    if (Main.serializer().descriptor.toString() != "Main(fields: MainFields)") return "Error3"
+    val boxListDesc = Box.serializerLike(String.serializer()).descriptor
+    if (boxListDesc.toString() != "kotlin.collections.ArrayList(Box(boxed: kotlin.String))") return boxListDesc.toString()
+    val boxDesc = Box.serializer(String.serializer()).descriptor
+    if (boxDesc.toString() != "Box(boxed: kotlin.String)") return boxDesc.toString()
+    return "OK"
+}