pgsql: Avoid global LC_CTYPE dependency in pg_locale_icu.c.

Avoid global LC_CTYPE dependency in pg_locale_icu.c.

ICU still depends on libc for compatibility with certain historical
behavior for single-byte encodings. Make the dependency explicit by
holding a locale_t object when required.

We should consider a better solution in the future, such as decoding
the text to UTF-32 and using u_tolower(). That would be a behavior
change and require additional infrastructure though; so for now, just
avoid the global LC_CTYPE dependency.

Reviewed-by: Chao Li <li(dot)evan(dot)chao(at)gmail(dot)com>
Discussion: https://postgr.es/m/450ceb6260cad30d7afdf155d991a9caafee7c0d.camel@j-davis.com

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/0a90df58cf38cf68d59c6841513be98aeeff250e

Modified Files
--------------
src/backend/utils/adt/pg_locale_icu.c | 47 ++++++++++++++++++++++++++++++++---
src/include/utils/pg_locale.h | 1 +
2 files changed, 44 insertions(+), 4 deletions(-)