[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

     if (IsSignBitSet()) {
       result.value = result.value.Negate().value;
     }
-    if (IsSignBitSet() != result.value.IsNegative()) {
-      result.flags.set(RealFlag::Overflow);
+    if (!result.value.IsZero()) {
+      if (IsSignBitSet() != result.value.IsNegative()) {
+        result.flags.set(RealFlag::Overflow);
+      }
     }
     if (result.flags.test(RealFlag::Overflow)) {
       result.value =