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/stdlib/jvm/build.gradle

@@ -174,7 +174,8 @@ compileKotlin {
                 "-Xallow-kotlin-package",
                 "-Xmultifile-parts-inherit",
                 "-Xnormalize-constructor-calls=enable",
-                "-module-name", "kotlin-stdlib"
+                "-module-name", "kotlin-stdlib",
+                "-Xuse-experimental=kotlin.Experimental"
         ]
     }
 }