[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

     position_ = pos;
     openPosition_.reset();
   }
+  void CloseFd(IoErrorHandler &);
 
   int fd_{-1};
   OwningPtr<char> path_;

flang/runtime/file.cpp

       (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'");

     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,

   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