[K2] Support legacy serialization for custom function type extensions.

For Compose libraries, it is important that libraries built with
K2 in version 2.0 can be read and used by a version 1.9 K1
compiler. That is not possible if custom function types are
in the metadata.

^KT-58456 Fixed

compiler/fir/fir-serialization/src/org/jetbrains/kotlin/fir/serialization/FirElementSerializer.kt

@@ -7,7 +7,9 @@ package org.jetbrains.kotlin.fir.serialization
 
 import org.jetbrains.kotlin.builtins.StandardNames
 import org.jetbrains.kotlin.builtins.functions.FunctionTypeKind
+import org.jetbrains.kotlin.builtins.functions.isBuiltin
 import org.jetbrains.kotlin.config.LanguageFeature
+import org.jetbrains.kotlin.config.LanguageVersion
 import org.jetbrains.kotlin.config.LanguageVersionSettings
 import org.jetbrains.kotlin.constant.EnumValue
 import org.jetbrains.kotlin.constant.IntValue
@@ -823,7 +825,8 @@ class FirElementSerializer private constructor(
                 return lowerBound
             }
             is ConeClassLikeType -> {
-                if (type.functionTypeKind(session) == FunctionTypeKind.SuspendFunction) {
+                val functionTypeKind = type.functionTypeKind(session)
+                if (functionTypeKind == FunctionTypeKind.SuspendFunction) {
                     val runtimeFunctionType = type.suspendFunctionTypeToFunctionTypeWithContinuation(
                         session, StandardClassIds.Continuation
                     )
@@ -831,6 +834,13 @@ class FirElementSerializer private constructor(
                     functionType.flags = Flags.getTypeFlags(true, false)
                     return functionType
                 }
+                if (functionTypeKind?.isBuiltin == false) {
+                    val legacySerializationUntil =
+                        LanguageVersion.fromVersionString(functionTypeKind.serializeAsFunctionWithAnnotationUntil)
+                    if (legacySerializationUntil != null && languageVersionSettings.languageVersion < legacySerializationUntil) {
+                        return typeProto(type.customFunctionTypeToSimpleFunctionType(session))
+                    }
+                }
                 fillFromPossiblyInnerType(builder, type)
                 if (type.hasContextReceivers) {
                     typeAnnotations.addIfNotNull(