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

[Eli Zaretskii]

> From: Stefan Monnier <monnier@IRO.UMontreal.CA>
> Date: Sat, 30 Jun 2018 09:18:23 -0400
> Cc: 31853@debbugs.gnu.org
> 
> So regardless of whether we can speed it up like Eli did just now (by
> only looping over the live buffers), the org-scan-tags code that
> let-binds it within a tight loop should be fixed.

I agree.