[flang][runtime] Fix REWIND after non-advancing data transfer

[klausler]

Summary

A REWIND of a unit that's in the middle of a record due to a READ
or WRITE statement with ADVANCE='NO' needs to reset the left tab
limit so that the next transfer takes place at the beginning of
the first record.

flang/runtime/unit.cpp

 
 void ExternalFileUnit::Rewind(IoErrorHandler &handler) {
   if (access == Access::Direct) {
     handler.SignalError(IostatRewindNonSequential,
         "REWIND(UNIT=%d) on non-sequential file", unitNumber());
   } else {
     SetPosition(0, handler);
     currentRecordNumber = 1;
+    leftTabLimit.reset();
   }
 }
 
 void ExternalFileUnit::SetPosition(std::int64_t pos, IoErrorHandler &handler) {
   DoImpliedEndfile(handler);
   frameOffsetInFile_ = pos;
   recordOffsetInFrame_ = 0;
   if (access == Access::Direct) {