bug#23833: 24.5; assoc-string with CASE-FOLD may fail

[Eli Zaretskii]

> From: Noam Postavsky <npostavs@users.sourceforge.net>
> Date: Thu, 23 Jun 2016 11:53:00 -0400
> Cc: ynyaaa@gmail.com, 23833@debbugs.gnu.org
> 
> Would it not make more sense if upcase converted dz into DZ?

We want to go by what UnicodeData.txt says:

  01F1;LATIN CAPITAL LETTER DZ;Lu;0;L;<compat> 0044 005A;;;;N;;;;01F3;01F2
  01F2;LATIN CAPITAL LETTER D WITH SMALL LETTER Z;Lt;0;L;<compat> 0044 
007A;;;;N;;;01F1;01F3;01F2
  01F3;LATIN SMALL LETTER DZ;Ll;0;L;<compat> 0064 007A;;;;N;;;01F1;;01F2

The problem here is that both DZ and Dz name dz as their lower-case
variant, and Emacs can only have one pair.  So we chose the other one
for the upcase conversion.  We could switch them, but one of them will
necessarily be lost, this way or another.