pgsql: Fix pg_iswcased() for some libc case mappings.

Fix pg_iswcased() for some libc case mappings.

Some C libraries, such as macOS libc, case-map characters that they do
not classify as upper or lower. ILIKE index scans could then miss
rows, because those characters stayed in the fixed prefix.

Introduced in 630706ced0. 43a948b7b25 fixed similar issues, but missed
this one.

Author: shihao zhong <zhong950419(at)gmail(dot)com>
Reviewed-by: Fujii Masao <masao(dot)fujii(at)gmail(dot)com>
Reviewed-by: Chao Li <li(dot)evan(dot)chao(at)gmail(dot)com>
Discussion: https://postgr.es/m/CAGRkXqS6pr-hG-g=K6RTdRw53SnMZWz3iXvh_wi8rqLSf972sA@mail.gmail.com
Backpatch-through: 19

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/8474bae2ba4cf8c6e91db800a441daffe22098ee

Modified Files
--------------
src/backend/utils/adt/pg_locale_libc.c | 8 ++++++--
1 file changed, 6 insertions(+), 2 deletions(-)