Fix lc_collate_is_c() when LC_COLLATE != LC_CTYPE.

Fix lc_collate_is_c() when LC_COLLATE != LC_CTYPE.

An unfortunate typo in commit 2d819a08a1 can cause wrong results when
the default collation provider is libc, LC_CTYPE=C, and LC_COLLATE is
a real locale. Users with this combination of settings must REINDEX
all affected indexes.

The same typo can also cause performance degradation when LC_COLLATE=C
and LC_CTYPE is a real locale.

Problem does not exist in master (due to refactoring), so fix only in
version 17.

Reported-by: Drew Callahan
Discussion: https://postgr.es/m/d5081a7f4f6d425c28dd69d1e09b2e78f149e726.camel@j-davis.com