[libcxx] Fix undefined behavior in forward_list

Summary:
This patch is similar to the <list> fix but it has a few differences. This patch doesn't use a `__link_pointer` typedef because we don't need to change the linked list pointers because `forward_list` never stores a  `__forward_begin_node` in the linked list itself. 

The issue with `forward_list` is that the iterators store pointers to `__forward_list_node` and not `__forward_begin_node`. This is incorrect because `before_begin()` and `cbefore_begin()` return iterators that point to a `__forward_begin_node`. This means we incorrectly downcast the `__forward_begin_node` pointer to a `__node_pointer`. This downcast itself is sometimes UB but it cannot be safely removed until ABI v2. The more common cause of UB is when we deference the downcast pointer. (for example `__ptr_->__next_`). This can be fixed without an ABI break by upcasting `__ptr_` before accessing it.

The fix is as follows:

1. Introduce a `__iter_node_pointer` typedef that works  similar to `__link_pointer` in the last patch. In ABI v2 it is always a typedef for `__begin_node_pointer`.
2. Change the `__before_begin()` method to return the correct pointer type (`__begin_node_pointer`),
    Previously it incorrectly downcasted  the  `__forward_begin_node` to a `__node_pointer`  so it could be used to constructor the iterator types.
3. Change `__forward_list_iterator` and `__forward_list_const_iterator`  in the following way:
    1. Change `__node_pointer __ptr_;` member to  have the `__iter_node_pointer` type instead. 
    2. Add additional private constructors that accept `__begin_node_pointer` in addition to `__node_pointer` and then correctly cast them to the stored `__iter_node_pointer` type. 
    3. Add  `__get_begin()` and `__get_node_unchecked()` accessor methods that correctly cast `__ptr_` to the expected pointer type. `__get_begin()` is always safe to use and should be 
       preferred. `__get_node_unchecked()` can only be used on a deferencible iterator.
4. Replace direct access to `__forward_list_iterator::__ptr_`  with the safe accessor methods.
  


Reviewers: mclow.lists, EricWF

Subscribers: cfe-commits

Differential Revision: http://reviews.llvm.org/D15836

llvm-svn: 258888

Author: EricWF

libcxx/include/__config

@@ -41,6 +41,7 @@
 #define _LIBCPP_ABI_INCOMPLETE_TYPES_IN_DEQUE
 // Fix undefined behavior in how std::list stores it's linked nodes.
 #define _LIBCPP_ABI_LIST_REMOVE_NODE_POINTER_UB
+#define _LIBCPP_ABI_FORWARD_LIST_REMOVE_NODE_POINTER_UB
 #endif
 
 #define _LIBCPP_CONCAT1(_LIBCPP_X,_LIBCPP_Y) _LIBCPP_X##_LIBCPP_Y