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

compiler/testData/multiplatform/optionalExpectation/jvm.kt

@@ -0,0 +1,6 @@
+actual annotation class A
+
+actual class C {
+    @A
+    actual fun f() {}
+}