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/cli/cli-common/src/org/jetbrains/kotlin/cli/common/arguments/CommonCompilerArguments.java

@@ -67,6 +67,9 @@ public abstract class CommonCompilerArguments {
     @Argument(value = "Xmulti-platform", description = "Enable experimental language support for multi-platform projects")
     public boolean multiPlatform;
 
+    @Argument(value = "Xno-check-impl", description = "Do not check presence of 'impl' modifier in multi-platform projects")
+    public boolean noCheckImpl;
+
     @Argument(value = "P", description = "Pass an option to a plugin")
     @ValueDescription(PLUGIN_OPTION_FORMAT)
     public String[] pluginOptions;