bug#31146: 27.0.50; ielm via cl-prin1 infloops when printing circular object

[Basil L. Contovounesios]

(This is a repeat of a recipe I sent to bug#18012 as a separate report.)

The following change:

81b5131e11: 2018-01-15 17:04:05 -0500
  * lisp/ielm.el (ielm-eval-input): Use cl-print.
  
https://git.savannah.gnu.org/cgit/emacs.git/commit/?id=81b5131e11fa96d32a1718c6e2ebf40994d7d281

now results in the following behaviour:

1. emacs -Q
2. M-x ielm RET
3. (let ((l (make-list 2 0)))
     (nconc l (cdr l)))
4. C-m
     => infloop
5. C-g
     => *** IELM Error ***  Quit during pretty-printing

Evaluating (4) in *scratch* via C-j, by contrast, inserts (0 0 . #1)
without inflooping, presumably because pp is used instead of cl-prin1.

-- 
Basil

In GNU Emacs 27.0.50 (build 1, x86_64-pc-linux-gnu, X toolkit, Xaw3d scroll 
bars)
 of 2018-04-12 built on thunk
Repository revision: 17174d3f77fe6d5103bfe7e858035a0038010522
Windowing system distributor 'The X.Org Foundation', version 11.0.11906000
System Description: Debian GNU/Linux buster/sid

Configured using:
 'configure --config-cache --prefix=/home/blc/.local --with-mailutils
 --with-x-toolkit=lucid --with-modules --with-file-notification=yes
 --with-x 'CFLAGS=-march=native -O2 -pipe''

Configured features:
XAW3D XPM JPEG TIFF GIF PNG RSVG IMAGEMAGICK SOUND GPM DBUS GSETTINGS
NOTIFY ACL LIBSELINUX GNUTLS LIBXML2 FREETYPE M17N_FLT LIBOTF XFT ZLIB
TOOLKIT_SCROLL_BARS LUCID X11 MODULES THREADS LIBSYSTEMD JSON LCMS2