JVM IR: retain optional annotations as package private classes

Similarly to how it's done in the old backend (see
PackageCodegenImpl.generateClassesAndObjectsInFile and
AnnotationCodegen.genAnnotation)

compiler/testData/writeFlags/multiplatform/optionalExpectation.kt

@@ -0,0 +1,12 @@
+// !LANGUAGE: +MultiPlatformProjects
+// !USE_EXPERIMENTAL: kotlin.ExperimentalMultiplatform
+// WITH_RUNTIME
+
+@file:Suppress("OPTIONAL_DECLARATION_USAGE_IN_NON_COMMON_SOURCE") // TODO: support common sources in the test infrastructure
+
+@OptionalExpectation
+expect annotation class Anno(val s: String)
+
+// TESTED_OBJECT_KIND: class
+// TESTED_OBJECTS: Anno
+// FLAGS: ACC_INTERFACE, ACC_ABSTRACT, ACC_ANNOTATION