Fix planner's test for case-foldable characters in ILIKE with ICU.

Fix planner's test for case-foldable characters in ILIKE with ICU.

As coded, the ICU-collation path in pattern_char_isalpha() failed
to consider regular ASCII letters to be case-varying.  This led to
like_fixed_prefix treating too much of an ILIKE pattern as being a
fixed prefix, so that indexscans derived from an ILIKE clause might
miss entries that they should find.

Per bug #15892 from James Inform.  This is an oversight in the original
ICU patch (commit eccfef81e), so back-patch to v10 where that came in.

Discussion: https://postgr.es/m/15892-e5d2bea3e8a04a1b@postgresql.org