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
diff --git a/include/iterator b/include/iterator
index 87d6d1d..8e92e6f 100644
--- a/include/iterator
+++ b/include/iterator
@@ -1197,12 +1197,13 @@
     _LIBCPP_INLINE_VISIBILITY iterator_type base() const _NOEXCEPT {return __i;}
 
 private:
-    _LIBCPP_INLINE_VISIBILITY __wrap_iter(iterator_type __x) _NOEXCEPT : __i(__x) {}
 #if _LIBCPP_DEBUG_LEVEL >= 2
     _LIBCPP_INLINE_VISIBILITY __wrap_iter(const void* __p, iterator_type __x) : __i(__x)
     {
         __get_db()->__insert_ic(this, __p);
     }
+#else
+    _LIBCPP_INLINE_VISIBILITY __wrap_iter(iterator_type __x) _NOEXCEPT : __i(__x) {}
 #endif
 
     template <class _Up> friend class __wrap_iter;