Output DWARF debug information for global 'using' declarations, instead

of just blowing up. A very similar change to this exists which is
GPLv3 licensed, this is my own change.

This problem was triggered by running the Boost regression tests.

See also:	http://gcc.gnu.org/bugzilla/show_bug.cgi?id=31899
Reviewed by:	luigi
Approved by:	re (kib)

contrib/gcc/dwarf2out.c

@@ -10007,7 +10007,7 @@ reference_to_unused (tree * tp, int * walk_subtrees,
     return NULL_TREE;
   else if (!cgraph_global_info_ready
 	   && (TREE_CODE (*tp) == VAR_DECL || TREE_CODE (*tp) == FUNCTION_DECL))
-    gcc_unreachable ();
+    return *tp;
   else if (DECL_P (*tp) && TREE_CODE (*tp) == VAR_DECL)
     {
       struct cgraph_varpool_node *node = cgraph_varpool_node (*tp);