[flang] Fix overflow detection for folding SUM/PRODUCT

[klausler]

Summary

The overflow detection code in the templates that fold SUM and PRODUCT
was checking for overflow before performing the reduction, not after.
Fix and add tests.

[PeteSteinfeld]

All builds and tests correctly and looks good.

flang/lib/Evaluate/fold-reduction.h

auto accumulator{[&](Element &element, const ConstantSubscripts &at) {
         overflow |= prod.SignedMultiplicationOverflowed();
         element = prod.lower;
       } else { // Real & Complex
         auto prod{element.Multiply(array->At(at))};
         overflow |= prod.flags.test(RealFlag::Overflow);
         element = prod.value;
       }
     }};
+    auto result{Expr<T>{DoReduction<T>(*array, dim, identity, accumulator)}};
     if (overflow) {
       context.messages().Say(
           "PRODUCT() of %s data overflowed"_warn_en_US, T::AsFortran());
-    } else {
-      return Expr<T>{DoReduction<T>(*array, dim, identity, accumulator)};
     }
+    return result;
   }
   return Expr<T>{std::move(ref)};
 }
 
 // SUM
 template <typename T>
 static Expr<T> FoldSum(FoldingContext &context, FunctionRef<T> &&ref) {
   static_assert(T::category == TypeCategory::Integer ||
auto accumulator{[&](Element &element, const ConstantSubscripts &at) {
         overflow |= sum.flags.test(RealFlag::Overflow);
         // correction = (sum - element) - next; algebraically zero
         correction = sum.value.Subtract(element, rounding)
                          .value.Subtract(next.value, rounding)
                          .value;
         element = sum.value;
       }
     }};
+    auto result{Expr<T>{DoReduction<T>(*array, dim, identity, accumulator)}};
     if (overflow) {
       context.messages().Say(
           "SUM() of %s data overflowed"_warn_en_US, T::AsFortran());
-    } else {
-      return Expr<T>{DoReduction<T>(*array, dim, identity, accumulator)};
     }
+    return result;
   }
   return Expr<T>{std::move(ref)};
 }
 
 } // namespace Fortran::evaluate
 #endif // FORTRAN_EVALUATE_FOLD_REDUCTION_H_