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

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.

Differential Revision: https://reviews.llvm.org/D114897