LibJS: Always collect all garbage when destroying Heap

When the Heap is going down, it's our last chance to run destructors,
so add a separate collector mode where we simply skip over the marking
phase and go directly to sweeping. This causes everything to get swept
and all live cells get destroyed.

This way, valgrind reports 0 leaks on exit. :^)

Libraries/LibJS/Heap/Heap.cpp

@@ -51,6 +51,7 @@ Heap::Heap(Interpreter& interpreter)
 
 Heap::~Heap()
 {
+    collect_garbage(CollectionType::CollectEverything);
 }
 
 Cell* Heap::allocate_cell(size_t size)
@@ -72,11 +73,13 @@ Cell* Heap::allocate_cell(size_t size)
     return cell;
 }
 
-void Heap::collect_garbage()
+void Heap::collect_garbage(CollectionType collection_type)
 {
-    HashTable<Cell*> roots;
-    gather_roots(roots);
-    mark_live_cells(roots);
+    if (collection_type == CollectionType::CollectGarbage) {
+        HashTable<Cell*> roots;
+        gather_roots(roots);
+        mark_live_cells(roots);
+    }
     sweep_dead_cells();
 }
 

Libraries/LibJS/Heap/Heap.h

@@ -53,7 +53,12 @@ public:
         return static_cast<T*>(memory);
     }
 
-    void collect_garbage();
+    enum class CollectionType {
+        CollectGarbage,
+        CollectEverything,
+    };
+
+    void collect_garbage(CollectionType = CollectionType::CollectGarbage);
 
     Interpreter& interpreter() { return m_interpreter; }
 

Userland/js.cpp

@@ -85,7 +85,5 @@ int main(int argc, char** argv)
     if (print_last_result)
         printf("%s\n", result.to_string().characters());
 
-    dbg() << "Collecting garbage on exit...";
-    interpreter.heap().collect_garbage();
     return 0;
 }