Lagom: Add a tool to verify correctness of the LibJS GC

This is implemented as a Clang frontend tool, and currently does two
things:
  - Ensure for all fields wrapped in {Nonnull,}GCPtr<T>, T inherits from
    JS::Cell
  - Ensure for all fields not wrapped in {Nonnull,}GCPtr, that the type
    does not inherit from JS::Cell (otherwise it should be wrapped in a
    Ptr class).

In the future, this tool could be extended further. For example, we may
consider validating all implementations of Cell::visit_impl.

Meta/Lagom/Tools/LibJSGCVerifier/src/CellsHandler.h

@@ -0,0 +1,30 @@
+/*
+ * Copyright (c) 2023, Matthew Olsson <mattco@serenityos.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#pragma once
+
+#include <clang/ASTMatchers/ASTMatchFinder.h>
+#include <clang/ASTMatchers/ASTMatchers.h>
+#include <clang/Tooling/Tooling.h>
+#include <unordered_set>
+
+class CollectCellsHandler
+    : public clang::tooling::SourceFileCallbacks
+    , public clang::ast_matchers::MatchFinder::MatchCallback {
+public:
+    CollectCellsHandler();
+    virtual ~CollectCellsHandler() override = default;
+
+    virtual bool handleBeginSource(clang::CompilerInstance&) override;
+
+    virtual void run(clang::ast_matchers::MatchFinder::MatchResult const& result) override;
+
+    clang::ast_matchers::MatchFinder& finder() { return m_finder; }
+
+private:
+    std::unordered_set<std::string> m_visited_classes;
+    clang::ast_matchers::MatchFinder m_finder;
+};