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

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.

Differential Revision: https://reviews.llvm.org/D127783

flang/runtime/unit.cpp

@@ -640,6 +640,7 @@ void ExternalFileUnit::Rewind(IoErrorHandler &handler) {
   } else {
     SetPosition(0, handler);
     currentRecordNumber = 1;
+    leftTabLimit.reset();
   }
 }