[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

if (IsConnected()) {
     if (!newPath.get() || isSamePath) {
       // OPEN of existing unit, STATUS='OLD' or unspecified, not new FILE=
       newPath.reset();
       return;
     }
     // 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) {
     if (const auto *already{
             GetUnitMap().LookUp(newPath.get(), newPathLength)}) {
       handler.SignalError(IostatOpenAlreadyConnected,
           "OPEN(UNIT=%d,FILE='%.*s'): file is already connected to unit %d",
           unitNumber_, static_cast<int>(newPathLength), newPath.get(),