pgsql: ltree: fix case-insensitive matching.

ltree: fix case-insensitive matching.

Previously, ltree_prefix_eq_ci() used lowercasing with the default
collation; while ltree_crc32_sz() used tolower() directly. These were
equivalent only if the default collation provider was libc and the
encoding was single-byte.

Change both to use casefolding with the default collation.

Backpatch through 18, where the casefolding APIs were introduced. The
bug exists in earlier versions, but would require some adaptation.

A REINDEX is required for ltree indexes where the database default
collation is not libc.

Reviewed-by: Chao Li <li(dot)evan(dot)chao(at)gmail(dot)com>
Reviewed-by: Peter Eisentraut <peter(at)eisentraut(dot)org>
Backpatch-through: 18
Discussion: https://postgr.es/m/450ceb6260cad30d7afdf155d991a9caafee7c0d.camel@j-davis.com
Discussion: https://postgr.es/m/01fc00fd66f641b9693d4f9f1af0ccf44cbdfbdf.camel@j-davis.com

Branch
------
REL_18_STABLE

Details
-------
https://git.postgresql.org/pg/commitdiff/806555e3000d0b0e0c536c1dc65548128d457d86

Modified Files
--------------
contrib/ltree/crc32.c | 47 +++++++++++++++++++++++++++++++++++--------
contrib/ltree/lquery_op.c | 39 ++++++++++++++++++++++++++++++++---
src/include/utils/pg_locale.h | 14 +++++++++++++
3 files changed, 89 insertions(+), 11 deletions(-)