gcc: warn about integer overflow in constant expressions in the C++ frontend.

This fixes GCC 28986:
 http://gcc.gnu.org/ml/gcc-patches/2007-01/msg00473.html

Obtained from:	gcc 4.3 (rev. 120558; GPLv2)
MFC after:	3 weeks

contrib/gcc/cp/typeck.c

@@ -3773,6 +3773,12 @@ build_binary_op (enum tree_code code, tree orig_op0, tree orig_op1,
   result = fold_if_not_in_template (result);
   if (final_type != 0)
     result = cp_convert (final_type, result);
+
+  if (TREE_OVERFLOW_P (result) 
+      && !TREE_OVERFLOW_P (op0) 
+      && !TREE_OVERFLOW_P (op1))
+    overflow_warning (result);
+
   return result;
 }