Fix multiple serialization plugin problems

Do not use fallback serializer if it is the same (fixes
https://github.com/Kotlin/kotlinx.serialization/issues/957).

Use correct number of serializable KClasses when creating sealed class
serializer (fixes https://github.com/Kotlin/kotlinx.serialization/issues/925).

plugins/kotlin-serialization/kotlin-serialization-compiler/src/org/jetbrains/kotlinx/serialization/compiler/backend/common/TypeUtil.kt

@@ -87,7 +87,9 @@ fun AbstractSerialGenerator.allSealedSerializableSubclassesFor(
     }
 
     val serializableSubtypes = recursiveSealed(klass).map { it.toSimpleType() }
-    return serializableSubtypes to serializableSubtypes.mapNotNull { findTypeSerializerOrContextUnchecked(module, it) }
+    return serializableSubtypes.mapNotNull { subtype ->
+        findTypeSerializerOrContextUnchecked(module, subtype)?.let { Pair(subtype, it) }
+    }.unzip()
 }
 
 fun KotlinType.serialName(): String {

plugins/kotlin-serialization/kotlin-serialization-compiler/src/org/jetbrains/kotlinx/serialization/compiler/backend/jvm/JVMCodegenUtil.kt

@@ -294,7 +294,7 @@ internal fun AbstractSerialGenerator.stackValueSerializerInstance(codegen: Class
                 if (serializer.classId == contextSerializerId && serializer.constructors.any { it.valueParameters.size == 3 }) {
                     // append new additional arguments
                     val fallbackDefaultSerializer = findTypeSerializer(module, kType)
-                    if (fallbackDefaultSerializer != null) {
+                    if (fallbackDefaultSerializer != null && fallbackDefaultSerializer != serializer) {
                         instantiate(kType to fallbackDefaultSerializer, writeSignature = false)
                     } else {
                         aconst(null)