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/codegen/box/jvmName/loadJvmName.kt

@@ -0,0 +1,13 @@
+// TARGET_BACKEND: JVM
+// WITH_RUNTIME
+
+@JvmName("bar")
+fun foo() {}
+
+fun getJvmName(): JvmName? =
+        Class.forName("LoadJvmNameKt").declaredMethods.single { it.name == "bar" }.getAnnotation(JvmName::class.java)
+
+fun box(): String {
+    // JvmName is binary-retained and should not be accessible via reflection
+    return if (getJvmName() == null) "OK" else "Fail"
+}

compiler/testData/codegen/box/multiplatform/optionalExpectation.kt

@@ -0,0 +1,49 @@
+// !LANGUAGE: +MultiPlatformProjects
+// !USE_EXPERIMENTAL: kotlin.ExperimentalMultiplatform
+// TARGET_BACKEND: JVM
+// WITH_RUNTIME
+// FILE: common.kt
+
+@OptionalExpectation
+expect annotation class Anno(val s: String)
+
+// FILE: jvm.kt
+
+import java.lang.reflect.AnnotatedElement
+
+@Anno("Foo")
+class Foo @Anno("<init>") constructor(@Anno("x") x: Int) {
+    @Anno("bar")
+    fun bar() {}
+
+    @Anno("getX")
+    var x = x
+        @Anno("setX")
+        set
+
+    @Anno("Nested")
+    interface Nested
+}
+
+private fun check(element: AnnotatedElement) {
+    check(element.annotations)
+}
+
+private fun check(annotations: Array<Annotation>) {
+    val filtered = annotations.filterNot { it.annotationClass.java.name == "kotlin.Metadata" }
+    if (filtered.isNotEmpty()) {
+        throw AssertionError("Annotations should be empty: $filtered")
+    }
+}
+
+fun box(): String {
+    val foo = Foo::class.java
+    check(foo)
+    check(Foo.Nested::class.java)
+    check(foo.declaredMethods.single { it.name == "bar" })
+    check(foo.declaredMethods.single { it.name == "getX" })
+    check(foo.declaredMethods.single { it.name == "setX" })
+    check(foo.constructors.single())
+    check(foo.constructors.single().parameterAnnotations.single())
+    return "OK"
+}