Introduce OptionalExpectation for annotations missing on some platforms

This commits adds a new annotation OptionalExpectation to the standard
library, which is experimental. To enable its usage, either pass
'-Xuse-experimental=kotlin.ExperimentalMultiplatform' as a compiler
argument, or '-Xuse-experimental=kotlin.Experimental' and also annotate
each usage with `@UseExperimental(ExperimentalMultiplatform::class)`

 #KT-18882 Fixed

libraries/tools/binary-compatibility-validator/reference-public-api/kotlin-stdlib-runtime-merged.txt

@@ -31,6 +31,9 @@ public final class kotlin/Experimental$Level : java/lang/Enum {
 	public static fun values ()[Lkotlin/Experimental$Level;
 }
 
+public abstract interface annotation class kotlin/ExperimentalMultiplatform : java/lang/annotation/Annotation {
+}
+
 public abstract interface annotation class kotlin/ExtensionFunctionType : java/lang/annotation/Annotation {
 }
 
@@ -96,6 +99,9 @@ public final class kotlin/NotImplementedError : java/lang/Error {
 	public synthetic fun <init> (Ljava/lang/String;ILkotlin/jvm/internal/DefaultConstructorMarker;)V
 }
 
+public abstract interface annotation class kotlin/OptionalExpectation : java/lang/annotation/Annotation {
+}
+
 public final class kotlin/Pair : java/io/Serializable {
 	public fun <init> (Ljava/lang/Object;Ljava/lang/Object;)V
 	public final fun component1 ()Ljava/lang/Object;