[flang] NINT(-.4999) is 0, not overflow

[klausler]

Summary

Overflow detection in the folding of int/nint/ceiling is
incorrectly signalling overflow when a negative argument yields
a zero result.

[jeanPerier]

LGTM

flang/include/flang/Evaluate/real.h

constexpr ValueWithRealFlags<INT> ToInteger(
     result.value = converted.value.SHIFTL(shift);
     if (converted.value.CompareUnsigned(result.value.SHIFTR(shift)) !=
         Ordering::Equal) {
       result.flags.set(RealFlag::Overflow);
     }
     if (IsSignBitSet()) {
       result.value = result.value.Negate().value;
     }
+    if (!result.value.IsZero()) {
       if (IsSignBitSet() != result.value.IsNegative()) {
         result.flags.set(RealFlag::Overflow);
       }
+    }
     if (result.flags.test(RealFlag::Overflow)) {
       result.value =
           IsSignBitSet() ? result.value.MASKL(1) : result.value.HUGE();
     }
     return result;
   }
 
   template <typename A>