Add KClass.isFun modifier to reflection

#KT-38881 Fixed

libraries/stdlib/jvm/runtime/kotlin/jvm/internal/ClassReference.kt

@@ -77,6 +77,10 @@ public class ClassReference(override val jClass: Class<*>) : KClass<Any>, ClassB
     override val isCompanion: Boolean
         get() = error()
 
+    @SinceKotlin("1.4")
+    override val isFun: Boolean
+        get() = error()
+
     private fun error(): Nothing = throw KotlinReflectionNotSupportedError()
 
     override fun equals(other: Any?) =

libraries/stdlib/jvm/src/kotlin/reflect/KClass.kt

@@ -127,6 +127,12 @@ public actual interface KClass<T : Any> : KDeclarationContainer, KAnnotatedEleme
     @SinceKotlin("1.1")
     public val isCompanion: Boolean
 
+    /**
+     * `true` if this class is a Kotlin functional interface.
+     */
+    @SinceKotlin("1.4")
+    public val isFun: Boolean
+
     /**
      * Returns `true` if this [KClass] instance represents the same Kotlin class as the class represented by [other].
      * On JVM this means that all of the following conditions are satisfied: