[flang] Don't close stderr in runtime (fixes STOP output)

[klausler]

Summary

STOP statement output was sometimes failing to appear because
the runtime flushes and shuts down open Fortran units beforehand.
But when file descriptor 2 was closed, the STOP statement output
was suppressed. The fix is to not actually close file descriptors
0-2 if they are connected to Fortran units being closed. This was
already the policy when an OPEN statement was (re-)opening such a
unit, so that logic has been pulled out into a member function and
shared with CLOSE processing.

flang/runtime/file.h

private:
   bool Seek(FileOffset, IoErrorHandler &);
   bool RawSeek(FileOffset);
   bool RawSeekToEnd();
   int PendingResult(const Terminator &, int);
   void SetPosition(FileOffset pos) {
     position_ = pos;
     openPosition_.reset();
   }
+  void CloseFd(IoErrorHandler &);
 
   int fd_{-1};
   OwningPtr<char> path_;
   std::size_t pathLength_;
   bool mayRead_{false};
   bool mayWrite_{false};
   bool mayPosition_{false};
   bool mayAsynchronous_{false};

flang/runtime/file.cpp

 }
 
 void OpenFile::Open(OpenStatus status, std::optional<Action> action,
     Position position, IoErrorHandler &handler) {
   if (fd_ >= 0 &&
       (status == OpenStatus::Old || status == OpenStatus::Unknown)) {
     return;
   }
-  if (fd_ >= 0) {
-    if (fd_ <= 2) {
-      // don't actually close a standard file descriptor, we might need it
-    } else {
-      if (::close(fd_) != 0) {
-        handler.SignalErrno();
-      }
-    }
-    fd_ = -1;
-  }
+  CloseFd(handler);
   if (status == OpenStatus::Scratch) {
     if (path_.get()) {
       handler.SignalError("FILE= must not appear with STATUS='SCRATCH'");
       path_.reset();
     }
     if (!action) {
       action = Action::ReadWrite;
     }
case CloseStatus::Keep:
     break;
   case CloseStatus::Delete:
     if (path_.get()) {
       ::unlink(path_.get());
     }
     break;
   }
   path_.reset();
-  if (fd_ >= 0) {
-    if (::close(fd_) != 0) {
-      handler.SignalErrno();
-    }
-    fd_ = -1;
-  }
+  CloseFd(handler);
 }
 
 std::size_t OpenFile::Read(FileOffset at, char *buffer, std::size_t minBytes,
     std::size_t maxBytes, IoErrorHandler &handler) {
   if (maxBytes == 0) {
     return 0;
   }
   CheckOpen(handler);

 }
 
 int OpenFile::PendingResult(const Terminator &terminator, int iostat) {
   int id{nextId_++};
   pending_ = New<Pending>{terminator}(id, iostat, std::move(pending_));
   return id;
 }
 
+void OpenFile::CloseFd(IoErrorHandler &handler) {
+  if (fd_ >= 0) {
+    if (fd_ <= 2) {
+      // don't actually close a standard file descriptor, we might need it
+    } else {
+      if (::close(fd_) != 0) {
+        handler.SignalErrno();
+      }
+    }
+    fd_ = -1;
+  }
+}
+
 bool IsATerminal(int fd) { return ::isatty(fd); }
 
 #ifdef WIN32
 // Access flags are normally defined in unistd.h, which unavailable under
 // Windows. Instead, define the flags as documented at
 // https://docs.microsoft.com/en-us/cpp/c-runtime-library/reference/access-waccess
 #define F_OK 00
 #define W_OK 02