Provide JvmSynthetic annotation in stdlib-common as optional

#KT-24478

libraries/stdlib/common/src/kotlin/JvmAnnotationsH.kt

@@ -64,6 +64,19 @@ public expect annotation class JvmMultifileClass()
 @OptionalExpectation
 public expect annotation class JvmField()
 
+/**
+ * Sets `ACC_SYNTHETIC` flag on the annotated target in the Java bytecode.
+ *
+ * Synthetic targets become inaccessible for Java sources at compile time while still being accessible for Kotlin sources.
+ * Marking target as synthetic is a binary compatible change, already compiled Java code will be able to access such target.
+ *
+ * This annotation is intended for *rare cases* when API designer needs to hide Kotlin-specific target from Java API
+ * while keeping it a part of Kotlin API so the resulting API is idiomatic for both languages.
+ */
+@Target(AnnotationTarget.FUNCTION, AnnotationTarget.PROPERTY_GETTER, AnnotationTarget.PROPERTY_SETTER, AnnotationTarget.FIELD)
+@OptionalExpectation
+public expect annotation class JvmSynthetic()
+
 /**
  * Instructs compiler to generate or omit wildcards for type arguments corresponding to parameters with
  * declaration-site variance, for example such as `Collection<out T>` has.

libraries/stdlib/jvm/runtime/kotlin/jvm/annotations/JvmPlatformAnnotations.kt

@@ -76,7 +76,7 @@ internal annotation class JvmPackageName(val name: String)
  */
 @Target(AnnotationTarget.FUNCTION, AnnotationTarget.PROPERTY_GETTER, AnnotationTarget.PROPERTY_SETTER, AnnotationTarget.FIELD)
 @Retention(AnnotationRetention.SOURCE)
-public annotation class JvmSynthetic
+public actual annotation class JvmSynthetic
 
 /**
  * This annotation indicates what exceptions should be declared by a function when compiled to a JVM method.