ClangPlugins: Add LLVM lit test suite

Author: https://github.com/mattco98
Commit: 46ee2b5f06
Pull-request: https://github.com/SerenityOS/serenity/pull/24027
Reviewed-by: https://github.com/ADKaster ✅
Reviewed-by: https://github.com/nico

Tests/ClangPlugins/LibJSGCTests/missing_visit_edges_method.cpp

@@ -0,0 +1,16 @@
+/*
+ * Copyright (c) 2024, Matthew Olsson <mattco@serenityos.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+// RUN: %clang++ -cc1 -verify %plugin_opts% %s 2>&1
+
+#include <LibJS/Runtime/Object.h>
+
+// expected-warning@+1 {{JS::Cell-inheriting class TestClass contains a GC-allocated member 'm_cell' but has no visit_edges method}}
+class TestClass : public JS::Object {
+    JS_OBJECT(TestClass, JS::Object);
+
+    JS::GCPtr<JS::Object> m_cell;
+};