[flang][runtime] Preserve effect of positioning in record in non-advancing output

[klausler]

Summary

When formatted non-advancing output ends in a control edit descriptor
like nX or Tn or TRn that effectively extends the record, fill any
gap with explicit blanks at the completion of the WRITE.

[clementval]

LGTM

flang/runtime/io-stmt.cpp

void ExternalIoStatementState<DIR>::CompleteOperation() {
   if (completedOperation()) {
     return;
   }
   if constexpr (DIR == Direction::Input) {
     BeginReadingRecord(); // in case there were no I/O items
     if (!mutableModes().nonAdvancing || GetIoStat() == IostatEor) {
       FinishReadingRecord();
     }
-  } else {
+  } else { // output
     if (mutableModes().nonAdvancing) {
+      // Make effects of positioning past the last Emit() visible with blanks.
+      std::int64_t n{unit().positionInRecord - unit().furthestPositionInRecord};
+      unit().positionInRecord = unit().furthestPositionInRecord;
+      while (n-- > 0 && unit().Emit(" ", 1, 1, *this)) {
+      }
       unit().leftTabLimit = unit().furthestPositionInRecord;
     } else {
       unit().leftTabLimit.reset();
       unit().AdvanceRecord(*this);
     }
     unit().FlushIfTerminal(*this);
   }
   return IoStatementBase::CompleteOperation();