bug#62162: 29.0.60; xref.el doesn't compile on Emacs 26, because multi-pair setq-local

[João Távora]

Hi, 

In a test run of the Eglot test suite (which tests Eglot on Emacs 26.3)

   
https://github.com/joaotavora/eglot/actions/runs/4405538673/jobs/7716553677#step:7:453

I'm reasonably sure that one of the failures has to do with a multi-pair
call to `setq-local` in xref.el which appeared in this commit

  commit 3573ebfa6d94411257ffefdf9eb72f508dbe502c
  Author: Juri Linkov <juri@linkov.net>
  Date:   Wed Nov 23 10:38:28 2022 +0200

That construct should not be used in xref.el because it isn't available
on Emacs 26, which xref purports to require.  In turn, this breaks Eglot
(and any other packge requiring xref) on those platforms.

João