Support "-Xno-check-impl" argument, check only real declarations

Use "-Xno-check-impl" to suppress checking whether the platform declaration
implementation has the "impl" modifier.

Do not check presence of fake overrides from platform class in the impl class,
otherwise there would be a lot of errors about the fact that
equals/hashCode/toString are not marked with the "impl" modifier

compiler/testData/cli/js/jsExtraHelp.out

@@ -4,6 +4,7 @@ where advanced options include:
   -Xrepeat <count>           Repeat compilation (for performance analysis)
   -Xplugin <path>            Load plugins from the given classpath
   -Xmulti-platform           Enable experimental language support for multi-platform projects
+  -Xno-check-impl            Do not check presence of 'impl' modifier in multi-platform projects
 
 Advanced options are non-standard and may be changed or removed without any notice.
 OK