Incorporate https://code-review.googlesource.com/#/c/re2/+/3050/ from
the re2 repository. Description of that change:

    Preserve the original behaviour of \s.

    Prior to Perl 5.18, \s did not match vertical tab. Bake that into
    make_perl_groups.pl as an override so that perl_groups.cc retains
    its current definitions when rebuilt with newer versions of Perl.

This fixes make_perl_groups.pl to generate an unchanged perl_groups.go
with perl versions 5.18 and later.

Fixes #22057

Change-Id: I9a56e9660092ed6c1ff1045b4a3847de355441a7
Reviewed-on: https://go-review.googlesource.com/103517
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>