LibGC: Add a ForeignCell class for ownership of non-C++ objects

This will allow us to use the GC to manage the lifetime of objects
that are not C++ objects, such as Swift objects. In the future we
could expand this cursed FFI to other languages as well.

Libraries/LibGC/ForeignCell.cpp

@@ -0,0 +1,61 @@
+/*
+ * Copyright (c) 2024, Andrew Kaster <andrew@ladybird.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#include <LibGC/DeferGC.h>
+#include <LibGC/ForeignCell.h>
+#include <LibGC/Heap.h>
+
+namespace GC {
+
+void* ForeignCell::foreign_data()
+{
+    // !!!
+    auto offset = round_up_to_power_of_two(sizeof(ForeignCell), m_vtable.alignment);
+    return static_cast<void*>(reinterpret_cast<u8*>(this) + offset);
+}
+
+ForeignCell::ForeignCell(ForeignCell::Vtable vtable)
+    : m_vtable(move(vtable))
+{
+    if (m_vtable.initialize)
+        m_vtable.initialize(foreign_data(), m_vtable.class_metadata_pointer, *this);
+}
+
+ForeignCell::~ForeignCell()
+{
+    if (m_vtable.destroy)
+        m_vtable.destroy(foreign_data(), m_vtable.class_metadata_pointer);
+}
+
+Ref<ForeignCell> ForeignCell::create(Heap& heap, size_t size, ForeignCell::Vtable vtable)
+{
+    // NOTE: GC must be deferred so that a collection during allocation doesn't get tripped
+    //   up looking for the Cell pointer on the stack or in a register when it might only exist in the heap.
+    //   We can't guarantee that the ForeignCell will be stashed in a proper ForeignRef/ForeignPtr or similar
+    //   foreign type until after all the dust has settled on both sides of the FFI boundary.
+    VERIFY(heap.is_gc_deferred());
+    VERIFY(is_power_of_two(vtable.alignment));
+    auto& allocator = heap.allocator_for_size(sizeof(ForeignCell) + round_up_to_power_of_two(size, vtable.alignment));
+    auto* memory = allocator.allocate_cell(heap);
+    auto* foreign_cell = new (memory) ForeignCell(move(vtable));
+    return *foreign_cell;
+}
+
+void ForeignCell::finalize()
+{
+    Base::finalize();
+    if (m_vtable.finalize)
+        m_vtable.finalize(foreign_data(), m_vtable.class_metadata_pointer);
+}
+
+void ForeignCell::visit_edges(Cell::Visitor& visitor)
+{
+    Base::visit_edges(visitor);
+    if (m_vtable.visit_edges)
+        m_vtable.visit_edges(foreign_data(), m_vtable.class_metadata_pointer, visitor);
+}
+
+}