[flang][runtime] Reset unit frame buffer when re-opening

[klausler]

Summary

An OPEN statement that implies closing a connection must invalidate
the unit's frame buffer so as to prevent stale data from the old
connection from being read into the newly-connected unit.

flang/runtime/unit.cpp

     // Otherwise, OPEN on open unit with new FILE= implies CLOSE
     DoImpliedEndfile(handler);
     FlushOutput(handler);
+    TruncateFrame(0, handler);
     Close(CloseStatus::Keep, handler);
   }
   if (newPath.get() && newPathLength > 0) {