FIR JS: Prepare JS/KLib infrastructure for FIR integration

rearrange declarations, abstract things that should be done differently
with FIR, etc.

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

@@ -56,21 +56,6 @@ class KlibMetadataIncrementalSerializer(
         return serializeDescriptors(fqName, classifierDescriptors, topLevelDescriptors).single()
     }
 
-    fun serializedMetadata(
-        fragments: Map<String, List<ByteArray>>,
-        header: ByteArray
-    ): SerializedMetadata {
-        val fragmentNames = mutableListOf<String>()
-        val fragmentParts = mutableListOf<List<ByteArray>>()
-
-        for ((fqName, fragment) in fragments.entries.sortedBy { it.key }) {
-            fragmentNames += fqName
-            fragmentParts += fragment
-        }
-
-        return SerializedMetadata(header, fragmentParts, fragmentNames)
-    }
-
     // TODO: For now, in the incremental serializer, we assume
     // there is only a single package fragment per file.
     // This is no always the case, actually.
@@ -78,4 +63,19 @@ class KlibMetadataIncrementalSerializer(
     // See monolithic serializer for details.
     override val TOP_LEVEL_DECLARATION_COUNT_PER_FILE = null
     override val TOP_LEVEL_CLASS_DECLARATION_COUNT_PER_FILE = null
+}
+
+fun makeSerializedKlibMetadata(
+    fragments: Map<String, List<ByteArray>>,
+    header: ByteArray
+): SerializedMetadata {
+    val fragmentNames = mutableListOf<String>()
+    val fragmentParts = mutableListOf<List<ByteArray>>()
+
+    for ((fqName, fragment) in fragments.entries.sortedBy { it.key }) {
+        fragmentNames += fqName
+        fragmentParts += fragment
+    }
+
+    return SerializedMetadata(header, fragmentParts, fragmentNames)
 }

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

@@ -253,22 +253,7 @@ abstract class KlibMetadataSerializer(
         fragmentNames: List<String>,
         emptyPackages: List<String>
     ): KlibMetadataProtoBuf.Header {
-        val header = KlibMetadataProtoBuf.Header.newBuilder()
-
-        header.moduleName = moduleDescriptor.name.asString()
-
-        if (languageVersionSettings.isPreRelease()) {
-            header.flags = 1
-        }
-
-        fragmentNames.forEach {
-            header.addPackageFragmentName(it)
-        }
-        emptyPackages.forEach {
-            header.addEmptyPackage(it)
-        }
-
-        return header.build()
+        return serializeKlibHeader(languageVersionSettings, moduleDescriptor, fragmentNames, emptyPackages)
     }
 
     // For platform libraries we get HUGE files.
@@ -278,6 +263,30 @@ abstract class KlibMetadataSerializer(
     abstract protected val TOP_LEVEL_CLASS_DECLARATION_COUNT_PER_FILE: Int?
 }
 
+fun serializeKlibHeader(
+    languageVersionSettings: LanguageVersionSettings,
+    moduleDescriptor: ModuleDescriptor,
+    fragmentNames: List<String>,
+    emptyPackages: List<String>
+): KlibMetadataProtoBuf.Header {
+    val header = KlibMetadataProtoBuf.Header.newBuilder()
+
+    header.moduleName = moduleDescriptor.name.asString()
+
+    if (languageVersionSettings.isPreRelease()) {
+        header.flags = 1
+    }
+
+    fragmentNames.forEach {
+        header.addPackageFragmentName(it)
+    }
+    emptyPackages.forEach {
+        header.addEmptyPackage(it)
+    }
+
+    return header.build()
+}
+
 fun DeclarationDescriptor.extractFileId(): Int? = when (this) {
     is DeserializedClassDescriptor -> classProto.getExtension(KlibMetadataProtoBuf.classFile)
     is DeserializedSimpleFunctionDescriptor -> proto.getExtension(KlibMetadataProtoBuf.functionFile)