Debug mode for string. This commit also marks the first time libc++ debug-mode has found a bug (found one in regex). Had to play with extern templates a bit to get this to work since string is heavily used within libc++.dylib.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@189114 91177308-0d34-0410-b5e6-96231b3b80d8

include/algorithm

@@ -1692,6 +1692,8 @@ __unwrap_iter(move_iterator<_Tp*> __i)
     return __i.base();
 }
 
+#if _LIBCPP_DEBUG_LEVEL < 2
+
 template <class _Tp>
 inline _LIBCPP_INLINE_VISIBILITY
 typename enable_if
@@ -1704,6 +1706,8 @@ __unwrap_iter(__wrap_iter<_Tp*> __i)
     return __i.base();
 }
 
+#endif  // _LIBCPP_DEBUG_LEVEL < 2
+
 template <class _InputIterator, class _OutputIterator>
 inline _LIBCPP_INLINE_VISIBILITY
 _OutputIterator