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/bytecodeListing/multiplatform/optionalExpectation.kt

@@ -0,0 +1,21 @@
+// !LANGUAGE: +MultiPlatformProjects
+// !USE_EXPERIMENTAL: kotlin.ExperimentalMultiplatform
+// TARGET_BACKEND: JVM
+// WITH_RUNTIME
+
+@OptionalExpectation
+expect annotation class Anno(val s: String)
+
+@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
+}

compiler/testData/codegen/bytecodeListing/multiplatform/optionalExpectation.txt

@@ -0,0 +1,15 @@
+@kotlin.Metadata
+public interface Foo$Nested {
+    inner class Foo$Nested
+}
+
+@kotlin.Metadata
+public final class Foo {
+    private field x: int
+    inner class Foo$Nested
+    public method <init>(p0: int): void
+    public final method bar(): void
+    public final method getX(): int
+    public final method setX(p0: int): void
+    public synthetic deprecated static method x$annotations(): void
+}