pgsql: Fix unintentional behavior change in commit e9931bfb75.

Fix unintentional behavior change in commit e9931bfb75.

Prior to that commit, there was special case to use ASCII case mapping
behavior for the libc provider with a single-byte encoding when that's
the default collation. Commit e9931bfb75 mistakenly eliminated that
special case; this commit restores it.

Discussion: https://postgr.es/m/01a104f0d2179d756261e90d96fd65c36ad6fcf0.camel@j-davis.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/e3fa2b037c6f0f435838e99200050dc54c306085

Modified Files
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src/backend/utils/adt/formatting.c | 30 +++++++++++++++++++++++++-----
src/backend/utils/adt/like.c | 2 ++
src/backend/utils/adt/pg_locale.c | 2 ++
src/include/utils/pg_locale.h | 1 +
4 files changed, 30 insertions(+), 5 deletions(-)