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/CMakeLists.txt

@@ -0,0 +1,19 @@
+cmake_minimum_required(VERSION 3.24)
+
+project(LibJSGCVerifier C CXX)
+
+find_package(Clang CONFIG REQUIRED HINTS "../../../../Toolchain/Local/clang")
+
+add_executable(LibJSGCVerifier src/main.cpp src/CellsHandler.cpp)
+
+target_include_directories(LibJSGCVerifier SYSTEM PRIVATE ${CLANG_INCLUDE_DIRS} ${LLVM_INCLUDE_DIRS})
+target_compile_features(LibJSGCVerifier PRIVATE cxx_std_20)
+target_link_libraries(LibJSGCVerifier PRIVATE clangTooling)
+
+target_compile_options(LibJSGCVerifier PRIVATE
+    -Wall
+    -Wextra
+    -Werror
+    -Wno-unused
+    -fno-rtti
+)