Added support of named companion in serializable class to the FIR

Merge-request: KT-MR-7472

plugins/kotlinx-serialization/testData/firMembers/serializableWithCompanion.kt

@@ -0,0 +1,56 @@
+// WITH_STDLIB
+
+package com.example
+
+import kotlinx.serialization.*
+import kotlinx.serialization.json.*
+import kotlinx.serialization.encoding.*
+import kotlinx.serialization.descriptors.*
+
+
+class NonSerializable {
+    companion object {
+        fun foo(): String {
+            return "OK"
+        }
+    }
+}
+
+@Serializable
+data class WithCompanion(val i: Int) {
+    companion object {
+        fun foo(): String {
+            return "OK"
+        }
+    }
+}
+
+@Serializable
+data class WithNamedCompanion(val i: Int) {
+    companion object Named {
+        fun foo(): String {
+            return "OK"
+        }
+    }
+}
+
+
+fun box(): String {
+    encodeAndDecode(WithCompanion.serializer(), WithCompanion(1), """{"i":1}""")?.let { return it }
+    encodeAndDecode(WithNamedCompanion.serializer(), WithNamedCompanion(2), """{"i":2}""")?.let { return it }
+    if (NonSerializable.foo() != "OK") return NonSerializable.foo()
+    if (WithCompanion.foo() != "OK") return WithCompanion.foo()
+    if (WithNamedCompanion.foo() != "OK") return WithNamedCompanion.foo()
+
+    return "OK"
+}
+
+
+private fun <T> encodeAndDecode(serializer: KSerializer<T>, value: T, expectedEncoded: String, expectedDecoded: T? = null): String? {
+    val encoded = Json.encodeToString(serializer, value)
+    if (encoded != expectedEncoded) return encoded
+
+    val decoded = Json.decodeFromString(serializer, encoded)
+    if (decoded != (expectedDecoded ?: value)) return "DECODED=${decoded.toString()}"
+    return null
+}