LibJS: Add JS_OBJECT_WITH_CUSTOM_CLASS_NAME

Some JS::Object subclasses need the GC-cell macro plumbing while still
computing their class_name() dynamically.

Add a GC_CELL helper that provides the Base alias and heap friendship
without defining class_name(), then expose it through LibJS. Teach the
LibJS GC Clang plugin to treat this helper as a JS_OBJECT-style macro so
the existing validation still checks the class name and base class
arguments.

Libraries/LibGC/Cell.h

@@ -38,6 +38,11 @@ public:                                            \
     }                                              \
     friend class GC::Heap;
 
+#define GC_CELL_WITH_CUSTOM_CLASS_NAME(class_, base_class) \
+public:                                                    \
+    using Base = base_class;                               \
+    friend class GC::Heap;
+
 class GC_API Cell {
     AK_MAKE_NONCOPYABLE(Cell);
     AK_MAKE_NONMOVABLE(Cell);

Libraries/LibJS/Runtime/Object.h

@@ -26,6 +26,7 @@
 namespace JS {
 
 #define JS_OBJECT(class_, base_class) GC_CELL(class_, base_class)
+#define JS_OBJECT_WITH_CUSTOM_CLASS_NAME(class_, base_class) GC_CELL_WITH_CUSTOM_CLASS_NAME(class_, base_class)
 
 struct PrivateElement {
     enum class Kind {

Meta/Lagom/ClangPlugins/LibJSGCPluginAction.cpp

@@ -944,6 +944,7 @@ void LibJSPPCallbacks::MacroExpands(clang::Token const& name_token, clang::Macro
         static llvm::StringMap<LibJSCellMacro::Type> libjs_macro_types {
             { "GC_CELL", LibJSCellMacro::Type::GCCell },
             { "JS_OBJECT", LibJSCellMacro::Type::JSObject },
+            { "JS_OBJECT_WITH_CUSTOM_CLASS_NAME", LibJSCellMacro::Type::JSObject },
             { "JS_ENVIRONMENT", LibJSCellMacro::Type::JSEnvironment },
             { "JS_PROTOTYPE_OBJECT", LibJSCellMacro::Type::JSPrototypeObject },
             { "WEB_PLATFORM_OBJECT", LibJSCellMacro::Type::WebPlatformObject },