[flang][runtime] Don't write implied ENDFILE for REC=/POS= (#79637)

An implied ENDFILE record, which truncates an external file, should be
written to a sequential unit whenever the file is repositioned for a
BACKSPACE or REWIND statement if a WRITE statement has executed since
the last OPEN/BACKSPACE/REWIND.

But the REC= and POS= positioning specifiers don't apply to sequential
units (they're for direct and stream units, resp.), so don't truncate
the file when they're used.

flang/runtime/unit.cpp

@@ -679,6 +679,7 @@ void ExternalFileUnit::Rewind(IoErrorHandler &handler) {
     handler.SignalError(IostatRewindNonSequential,
         "REWIND(UNIT=%d) on non-sequential file", unitNumber());
   } else {
+    DoImpliedEndfile(handler);
     SetPosition(0, handler);
     currentRecordNumber = 1;
     leftTabLimit.reset();
@@ -687,7 +688,6 @@ void ExternalFileUnit::Rewind(IoErrorHandler &handler) {
 }
 
 void ExternalFileUnit::SetPosition(std::int64_t pos, IoErrorHandler &handler) {
-  DoImpliedEndfile(handler);
   frameOffsetInFile_ = pos;
   recordOffsetInFrame_ = 0;
   if (access == Access::Direct) {
@@ -707,6 +707,12 @@ bool ExternalFileUnit::SetStreamPos(
         "POS=%zd is invalid", static_cast<std::intmax_t>(oneBasedPos));
     return false;
   }
+  // A backwards POS= implies truncation after writing, at least in
+  // Intel and NAG.
+  if (static_cast<std::size_t>(oneBasedPos - 1) <
+      frameOffsetInFile_ + recordOffsetInFrame_) {
+    DoImpliedEndfile(handler);
+  }
   SetPosition(oneBasedPos - 1, handler);
   // We no longer know which record we're in.  Set currentRecordNumber to
   // a large value from whence we can both advance and backspace.