Revert "[libc++] Fix incorrect down cast in __tree::operator="

This reverts commit 35110445081152f7f2d2a9d053bb6fa718216d7b.

I've accidentally pushed to the wrong branch.
diff --git a/libcxx/include/__tree b/libcxx/include/__tree
index 64d1436..6ca1a62 100644
--- a/libcxx/include/__tree
+++ b/libcxx/include/__tree
@@ -1388,7 +1388,7 @@
     if (__root())
       __root()->__parent_ = __end_node();
   }
-  __begin_node_ = static_cast<__end_node_pointer>(std::__tree_min(__end_node()->__left_));
+  __begin_node_ = static_cast<__end_node_pointer>(std::__tree_min(static_cast<__node_base_pointer>(__end_node())));
   __size_       = __t.size();
 
   return *this;