Speed up bootstrap __build_class__ a little bit

Since we're iterating over the layouts table one by one, and the layouts
table will always be at least LayoutId::kLastBuiltinId layouts long, use
`Runtime::layoutAt` instead of `Runtime::layoutAtSafe`, which does a
bunch of checks. This search is still linear and kind of long, though.

runtime/type-builtins.cpp

@@ -82,8 +82,16 @@ RawObject findBuiltinTypeWithName(Thread* thread, const Object& name) {
   Object layout(&scope, NoneType::object());
   Object type_obj(&scope, NoneType::object());
   for (int i = 0; i <= static_cast<int>(LayoutId::kLastBuiltinId); i++) {
-    layout = runtime->layoutAtSafe(static_cast<LayoutId>(i));
-    if (layout.isErrorNotFound()) continue;
+    LayoutId id = static_cast<LayoutId>(i);
+    if (id == LayoutId::kError) {
+      // layoutAt(LayoutId::kError) will fail.
+      continue;
+    }
+    layout = runtime->layoutAt(id);
+    if (layout == SmallInt::fromWord(0)) {
+      // Indicates an invalid LayoutId.
+      continue;
+    }
     type_obj = Layout::cast(*layout).describedType();
     if (!type_obj.isType()) continue;
     if (Type::cast(*type_obj).name() == name) {