Generate 'Deprecated' annotation on invisible companion object field

Design decision: if the field for a companion object should have
non-public visibility, generate it with @Deprecated annotation in
language version 1.2.

TODO: generate it with proper visibility in 1.3 and later.

KT-11567 Companion object INSTANCE field more visible than companion object class itself

compiler/util/src/org/jetbrains/kotlin/config/LanguageVersionSettings.kt

@@ -52,6 +52,7 @@ enum class LanguageFeature(
     ArrayLiteralsInAnnotations(KOTLIN_1_2),
     InlineDefaultFunctionalParameters(KOTLIN_1_2),
     SoundSmartCastsAfterTry(KOTLIN_1_2),
+    DeprecatedFieldForInvisibleCompanionObject(KOTLIN_1_2),
 
     // Experimental features