[flang] Fix buffering read->write transition

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.

Differential revision: https://reviews.llvm.org/D88612

flang/runtime/buffer.h

@@ -94,7 +94,7 @@ template <typename STORE> class FileFrame {
         start_ + (at - fileOffset_) + static_cast<std::int64_t>(bytes) >
             size_) {
       Flush(handler);
-      fileOffset_ = at;
+      Reset(at);
       Reallocate(bytes, handler);
     }
     dirty_ = true;

flang/runtime/io-api.cpp

@@ -518,7 +518,7 @@ bool IONAME(SetRec)(Cookie cookie, std::int64_t rec) {
   }
   connection.currentRecordNumber = rec;
   if (auto *unit{io.GetExternalFileUnit()}) {
-    unit->SetPosition(rec * *connection.recordLength);
+    unit->SetPosition((rec - 1) * *connection.recordLength);
   }
   return true;
 }