[flang][runtime] Fix directed UP/DOWN rounding edge case for Fw.d output

[klausler]

Summary

When Fw.d output editing takes place with directed rounding, make sure that
nonzero values that would normally be converted to zero round up (or down,
depending on the sign) to a scaled 1.

flang/runtime/edit-output.cpp

while (true) {
     if (expo > extraDigits && extraDigits >= 0 && canIncrease) {
       extraDigits = expo;
       if (!edit.digits.has_value()) { // F0
         fracDigits = sizeof buffer_ - extraDigits - 2; // sign & NUL
       }
       canIncrease = false; // only once
       continue;
     } else if (expo == -fracDigits && convertedDigits > 0) {
-      if (rounding != decimal::FortranRounding::RoundToZero) {
-        // Convert again without rounding so that we can round here
-        rounding = decimal::FortranRounding::RoundToZero;
-        continue;
-      } else if (converted.str[signLength] >= '5') {
-        // Value rounds up to a scaled 1 (e.g., 0.06 for F5.1 -> 0.1)
+      if ((rounding == decimal::FortranRounding::RoundUp &&
+              *converted.str != '-') ||
+          (rounding == decimal::FortranRounding::RoundDown &&
+              *converted.str == '-') ||
+          (rounding == decimal::FortranRounding::RoundToZero &&
+              rounding != edit.modes.round && // it changed below
+              converted.str[signLength] >= '5')) {
+        // Round up/down to a scaled 1
         ++expo;
         convertedDigits = 0;
         trailingOnes = 1;
+      } else if (rounding != decimal::FortranRounding::RoundToZero) {
+        // Convert again with truncation so first digit can be checked
+        // on the next iteration by the code above
+        rounding = decimal::FortranRounding::RoundToZero;
+        continue;
       } else {
         // Value rounds down to zero
         expo = 0;
         convertedDigits = 0;
       }
     } else if (expo < extraDigits && extraDigits > -fracDigits) {
       extraDigits = std::max(expo, -fracDigits);
       continue;