pgsql: Fix pg_iswcased().

Fix pg_iswcased().

Commits 630706ced0 and 9c8de15969 lost some encoding-specific nuances:
in libc with EUC encodings, or in ICU with any non-UTF8 encoding, we
cannot easily classify characters, so we must assume they are
case-varying. Restore these behaviors.

Previously, ILIKE using an index scan could cause wrong results.

Reported-by: Noah Misch <noah(at)leadboat(dot)com>
Discussion: https://postgr.es/m/20260826221526.89.noahmisch@microsoft.com
Backpatch-through: 19

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/43a948b7b252344a79eb49b9f3c5c8832bee19a9

Modified Files
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src/backend/utils/adt/pg_locale.c | 5 +++++
src/backend/utils/adt/pg_locale_icu.c | 16 ++++++++++++++-
src/backend/utils/adt/pg_locale_libc.c | 36 ++++++++++++++++++++++++++++++----
3 files changed, 52 insertions(+), 5 deletions(-)