Added support of external serializers to the FIR

Co-authored-by: Leonid Startsev <leonid.startsev@jetbrains.com>
Merge-request: KT-MR-7334
Merged-by: Sergey Shanshin <Sergey.Shanshin@jetbrains.com>

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

@@ -0,0 +1,27 @@
+// WITH_STDLIB
+
+package com.example
+
+import kotlinx.serialization.*
+import kotlinx.serialization.json.*
+import kotlinx.serialization.encoding.*
+import kotlinx.serialization.descriptors.*
+
+
+class Data(val j: Int)
+
+@Serializer(forClass = Data::class)
+object ObjectSerializer
+
+@Serializer(forClass = Data::class)
+class ClassSerializer
+
+fun box(): String {
+    val encodedForClass = Json.encodeToString(ClassSerializer(), Data(1))
+    if (encodedForClass != """{"j":1}""") return encodedForClass
+
+    val encodedForObject = Json.encodeToString(ObjectSerializer, Data(2))
+    if (encodedForObject != """{"j":2}""") return encodedForObject
+
+    return "OK"
+}