Introduce an internal attribute '@_show_in_interface' to be used in stdlib for underscored protocols that

should be shown in the interface.

Also switch the option and hide all underscored protocols by default, unless they are marked with the new attribute.

tools/SourceKit/lib/SwiftLang/SwiftEditor.cpp

@@ -1133,6 +1133,8 @@ std::vector<UIdent> UIDsFromDeclAttributes(const DeclAttributes &Attrs) {
     // We handle accessibility explicitly.
     case DAK_Accessibility:
     case DAK_SetterAccessibility:
+    // Ignore these.
+    case DAK_ShowInInterface:
       continue;
     default:
       break;