flang] Fix buffering read->write transition

[klausler]

Summary

The buffer needs to be Reset() after a Flush(), since the
Flush() can be a no-op after a read->write transition.
And record numbers are 1-based, not 0-based.
This fixes a bug with rewrites of records that have been
recently read.

flang/runtime/buffer.h

std::size_t ReadFrame(
     return FrameLength();
   }
 
   void WriteFrame(FileOffset at, std::size_t bytes, IoErrorHandler &handler) {
     if (!dirty_ || at < fileOffset_ || at > fileOffset_ + length_ ||
         start_ + (at - fileOffset_) + static_cast<std::int64_t>(bytes) >
             size_) {
       Flush(handler);
-      fileOffset_ = at;
+      Reset(at);
       Reallocate(bytes, handler);
     }
     dirty_ = true;
     frame_ = at - fileOffset_;
     length_ = std::max<std::int64_t>(length_, frame_ + bytes);
   }
 
   void Flush(IoErrorHandler &handler) {

flang/runtime/io-api.cpp

bool IONAME(SetRec)(Cookie cookie, std::int64_t rec) {
   }
   if (rec < 1) {
     io.GetIoErrorHandler().SignalError(
         "REC=%zd is invalid", static_cast<std::intmax_t>(rec));
     return false;
   }
   connection.currentRecordNumber = rec;
   if (auto *unit{io.GetExternalFileUnit()}) {
-    unit->SetPosition(rec * *connection.recordLength);
+    unit->SetPosition((rec - 1) * *connection.recordLength);
   }
   return true;
 }
 
 bool IONAME(SetRound)(Cookie cookie, const char *keyword, std::size_t length) {
   IoStatementState &io{*cookie};
   ConnectionState &connection{io.GetConnectionState()};
   static const char *keywords[]{"UP", "DOWN", "ZERO", "NEAREST", "COMPATIBLE",