bug#31853: 27.0.50; binding case-fold-search is slow when there are many buffers

[Eli Zaretskii]

> From: Noam Postavsky <npostavs@gmail.com>
> Date: Fri, 15 Jun 2018 22:17:44 -0400
> Cc: 31853@debbugs.gnu.org
> 
> Although my understanding of the resolution of Bug#18522 is that dead
> buffers should no longer affect the time for let-binding variables, just
> live ones (but maybe the fix missed something).

Which fix is that?  I think I'm missing it, because I don't think that
bug resolved the general problem.  Maybe I'm forgetting something.