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/common.kt

@@ -0,0 +1,10 @@
+// WITH_RUNTIME
+// ADDITIONAL_COMPILER_ARGUMENTS: -Xuse-experimental=kotlin.ExperimentalMultiplatform
+
+@OptionalExpectation
+expect annotation class A()
+
+expect class C {
+    @A
+    fun f()
+}

compiler/testData/multiplatform/optionalExpectation/js.kt

@@ -0,0 +1,3 @@
+actual class C {
+    actual fun f() {}
+}

compiler/testData/multiplatform/optionalExpectation/jvm.kt

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

compiler/testData/multiplatform/optionalExpectation/output.txt

@@ -0,0 +1,12 @@
+-- Common --
+Exit code: OK
+Output:
+
+-- JVM --
+Exit code: OK
+Output:
+
+-- JS --
+Exit code: OK
+Output:
+