pgsql: Fix pg_strupper/lower/title/fold() functions to work with C loca

Fix pg_strupper/lower/title/fold() functions to work with C locale

Starting with commit 011384ba45, calling to_date() or to_timestamp()
with "TMMonth" or other TM-prefixed format keyword, with the C locale,
would crash.

In REL_19_STABLE and above, pg_strupper(), pg_strlower(),
pg_strtitle(), and pg_strfold() functions have a special case for the
C locale, but that was missing in REL_18_STABLE. On REL_18_STABLE,
the functions call the libc function even in C locale, even though the
native locale object is NULL. On Linux, the underlying libc functions
will crash when called with NULL locale. (On macOS, they reportedly
do not, but even then it's not clear if they will do what you'd
expect.)

This went unnoticed because until commit 011384ba45, we never called
these functions in C locale, all the callers had a special codepath
for C locale. We could add a special path in the new callers too, but
it's an accident waiting to happen, so let's backport the C
locale-specific handling from REL_19_STABLE to REL_18_STABLE. Older
versions did not have these functions at all, hence no problem.

This applies to REL_18_STABLE only.

Reported-by: Masashi Kamura <kamura(dot)masashi(at)fujitsu(dot)com>
Reviewed-by: Álvaro Herrera <alvherre(at)kurilemu(dot)de>
Discussion: https://www.postgresql.org/message-id/OS9PR01MB1317436E07D06281AD1A0452F94DD2%40OS9PR01MB13174.jpnprd01.prod.outlook.com

Branch
------
REL_18_STABLE

Details
-------
https://git.postgresql.org/pg/commitdiff/5f003855e7f05bd03c5531a58838540f62b8b19e

Modified Files
--------------
src/backend/utils/adt/pg_locale.c | 68 ++++++++++++++++++++++++++++++++++++---
1 file changed, 64 insertions(+), 4 deletions(-)