Report "declaration should be marked with impl" when possible

Also support a quick fix to add 'impl' modifier (KT-18454), although it
doesn't work yet on classes because there's no error on them in the IDE

 #KT-18087 Fixed
 #KT-18452 Fixed
 #KT-18454

idea/src/org/jetbrains/kotlin/idea/quickfix/QuickFixRegistrar.kt

@@ -477,6 +477,7 @@ class QuickFixRegistrar : QuickFixContributor {
         OVERLOADS_WITHOUT_DEFAULT_ARGUMENTS.registerFactory(RemoveAnnotationFix.JvmOverloads)
 
         HEADER_WITHOUT_IMPLEMENTATION.registerFactory(CreateHeaderImplementationFix)
+        IMPL_MISSING.registerFactory(AddModifierFix.createFactory(KtTokens.IMPL_KEYWORD))
 
         CAST_NEVER_SUCCEEDS.registerFactory(ReplacePrimitiveCastWithNumberConversionFix)