Ada front-end depends on signed overflow

[Florian Weimer]

* Andrew Pinski:

> The Ada front-end is still being missed compiled by VRP but VRP is doing
> the correct thing as the type is signed and overflow on signed is 
> undefined
> (-fwrapv is not turned on by default for Ada).

It probably makes sense to turn on -fwrapv for Ada because even
without -gnato, the behavior is not really undefined:

| The reason that we distinguish overflow checking from other kinds of
| range constraint checking is that a failure of an overflow check can
| generate an incorrect value, but cannot cause erroneous behavior.

<http://gcc.gnu.org/onlinedocs/gcc-4.0.0/gnat_ugn_unw/Run_002dTime-Checks.html>

(Without -fwrapv, integer overflow is undefined, and subsequent range
checks can be optimized away, so that it might cause erroneous
behavior.)