Extensible mechanism for plugin metadata during descriptor serialization

A mechanism that allows kotlinx.serialization plugin to preserve the
correct (program) order of properties after serializing/deserializing
descriptors to kotlin metadata, which is needed for correct and stable
json serialization of class hierarchies in incremental/multi-module scenario.
It uses protobuf extensions.

compiler/ir/serialization.common/src/org/jetbrains/kotlin/backend/common/serialization/metadata/KlibMetadataIncrementalSerializer.kt

@@ -1,5 +1,5 @@
 /*
- * Copyright 2010-2019 JetBrains s.r.o. and Kotlin Programming Language contributors.
+ * Copyright 2010-2020 JetBrains s.r.o. and Kotlin Programming Language contributors.
  * Use of this source code is governed by the Apache 2.0 license that can be found in the license/LICENSE.txt file.
  */
 

compiler/ir/serialization.common/src/org/jetbrains/kotlin/backend/common/serialization/metadata/KlibMetadataMonolithicSerializer.kt

@@ -1,3 +1,8 @@
+/*
+ * Copyright 2010-2020 JetBrains s.r.o. and Kotlin Programming Language contributors.
+ * Use of this source code is governed by the Apache 2.0 license that can be found in the license/LICENSE.txt file.
+ */
+
 package org.jetbrains.kotlin.backend.common.serialization.metadata
 
 import org.jetbrains.kotlin.config.LanguageVersionSettings

compiler/ir/serialization.common/src/org/jetbrains/kotlin/backend/common/serialization/metadata/KlibMetadataSerializer.kt

@@ -1,5 +1,5 @@
 /*
- * Copyright 2010-2019 JetBrains s.r.o. and Kotlin Programming Language contributors.
+ * Copyright 2010-2020 JetBrains s.r.o. and Kotlin Programming Language contributors.
  * Use of this source code is governed by the Apache 2.0 license that can be found in the license/LICENSE.txt file.
  */
 
@@ -98,9 +98,11 @@ abstract class KlibMetadataSerializer(
             val index = classSerializer.stringTable.getFqNameIndex(classDescriptor)
             //builder.addExtension(KlibMetadataProtoBuf.className, index)
 
-            val classes = serializeClasses(packageName/*, builder*/,
+            val classes = serializeClasses(
+                packageName/*, builder*/,
                 classDescriptor.unsubstitutedInnerClassesScope
-                    .getContributedDescriptors(DescriptorKindFilter.CLASSIFIERS))
+                    .getContributedDescriptors(DescriptorKindFilter.CLASSIFIERS)
+            )
 
             classSerializer = previousSerializer
             return classes + Pair(classProto, index)