[libcxx] Constrain unique_ptr::operator=(unique_ptr<Tp, Dp>) in C++03 mode

[EricWF]

Summary

This patch properly constrains the converting assignment operator in C++03. It also fixes a bug where std::forward was given the wrong type.
The following two tests begin passing in C++03:

• unique_ptr.single.asgn/move_convert.pass.cpp
• unique_ptr.single.asgn/move_convert13.fail.cpp

[EricWF]

Update diff so it has more context.

[mclow.lists]

This is just making the two halves of the #ifdef consistent. LGTM.

include/memory

     {
         return __rv<unique_ptr>(*this);
     }
 
     _LIBCPP_INLINE_VISIBILITY unique_ptr(__rv<unique_ptr> __u)
         : __ptr_(__u->release(), _VSTD::forward<deleter_type>(__u->get_deleter())) {}
 
     template <class _Up, class _Ep>
-    _LIBCPP_INLINE_VISIBILITY unique_ptr& operator=(unique_ptr<_Up, _Ep> __u)
+    _LIBCPP_INLINE_VISIBILITY
+    typename enable_if<
+        !is_array<_Up>::value &&
+        is_convertible<typename unique_ptr<_Up, _Ep>::pointer, pointer>::value &&
+        is_assignable<deleter_type&, _Ep&>::value,
+        unique_ptr&
+    >::type
+    operator=(unique_ptr<_Up, _Ep> __u)
     {
         reset(__u.release());
-        __ptr_.second() = _VSTD::forward<deleter_type>(__u.get_deleter());
+        __ptr_.second() = _VSTD::forward<_Ep>(__u.get_deleter());
         return *this;
     }
 
     _LIBCPP_INLINE_VISIBILITY unique_ptr(pointer __p, deleter_type __d)
         : __ptr_(_VSTD::move(__p), _VSTD::move(__d)) {}
 
     template <class _Up>
         _LIBCPP_INLINE_VISIBILITY