[flang] Fix Gw.d format output

[klausler]

Summary

The estimation of the decimal exponent needs to allow for all
'd' of the requested significant digits.

Also accept a plus sign on a "+kP" scaling factor in a format.

[PeteSteinfeld]

All builds, tests, and looks good.

flang/runtime/edit-output.cpp

return EmitPrefix(edit, totalLength, width) &&
         io_.EmitRepeated(' ', trailingBlanks_) && EmitSuffix(edit);
   }
 }
 
 // 13.7.5.2.3 in F'2018
 template <int binaryPrecision>
 DataEdit RealOutputEditing<binaryPrecision>::EditForGOutput(DataEdit edit) {
   edit.descriptor = 'E';
-  if (!edit.width.has_value() ||
-      (*edit.width > 0 && edit.digits.value_or(-1) == 0)) {
+  int significantDigits{
+      edit.digits.value_or(BinaryFloatingPoint::decimalPrecision)}; // 'd'
+  if (!edit.width.has_value() || (*edit.width > 0 && significantDigits == 0)) {
     return edit; // Gw.0 -> Ew.0 for w > 0
   }
-  decimal::ConversionToDecimalResult converted{Convert(1, edit)};
+  decimal::ConversionToDecimalResult converted{
+      Convert(significantDigits, edit)};
   if (IsInfOrNaN(converted)) {
     return edit;
   }
   int expo{IsZero() ? 1 : converted.decimalExponent}; // 's'
-  int significantDigits{
-      edit.digits.value_or(BinaryFloatingPoint::decimalPrecision)}; // 'd'
   if (expo < 0 || expo > significantDigits) {
     return edit; // Ew.d
   }
   edit.descriptor = 'F';
   edit.modes.scale = 0; // kP is ignored for G when no exponent field
   trailingBlanks_ = 0;
   int editWidth{edit.width.value_or(0)};
   if (editWidth > 0) {

flang/runtime/format-implementation.h

int FormatControl<CONTEXT>::GetIntField(
   CharType ch{firstCh ? firstCh : PeekNext()};
   if (ch != '-' && ch != '+' && (ch < '0' || ch > '9')) {
     handler.SignalError(IostatErrorInFormat,
         "Invalid FORMAT: integer expected at '%c'", static_cast<char>(ch));
     return 0;
   }
   int result{0};
   bool negate{ch == '-'};
-  if (negate) {
+  if (negate || ch == '+') {
     firstCh = '\0';
     ch = PeekNext();
   }
   while (ch >= '0' && ch <= '9') {
     if (result >
         std::numeric_limits<int>::max() / 10 - (static_cast<int>(ch) - '0')) {
       handler.SignalError(
           IostatErrorInFormat, "FORMAT integer field out of range");