[flang][runtime] Refine list-directed REAL(2) output

[klausler]

Summary

The rule used by list-directed REAL output editing to select
between Ew.d and Fw.d output editing breaks down for 16-bit
floating-point data, since the number of significant decimal
digits is so low that Ew,d output editing is nearly always selected.
Cap the test so that five-digit values will be output with Fw.d
editing.

flang/runtime/edit-output.cpp

 template <int binaryPrecision>
 bool RealOutputEditing<binaryPrecision>::EditListDirectedOutput(
     const DataEdit &edit) {
   decimal::ConversionToDecimalResult converted{Convert(1, edit.modes.round)};
   if (IsInfOrNaN(converted)) {
     return EditEorDOutput(edit);
   }
   int expo{converted.decimalExponent};
-  if (expo < 0 || expo > BinaryFloatingPoint::decimalPrecision) {
+  // The decimal precision of 16-bit floating-point types is very low,
+  // so use a reasonable cap of 6 to allow more values to be emitted
+  // with Fw.d editing.
+  static constexpr int maxExpo{
+      std::max(6, BinaryFloatingPoint::decimalPrecision)};
+  if (expo < 0 || expo > maxExpo) {
     DataEdit copy{edit};
     copy.modes.scale = 1; // 1P
     return EditEorDOutput(copy);
   }
   return EditFOutput(edit);
 }
 
 // 13.7.5.2.6 in F'2018