pgsql: Const-ify a couple of datetime parsing subroutines.

Const-ify a couple of datetime parsing subroutines.

More could be done in this line, but I just grabbed some low-hanging
fruit.  Principal objective was to remove the need for several ugly
unconstify() usages in formatting.c.

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/bad5116957eea2dc360c6c233be08284bd3d5364

Modified Files
--------------
src/backend/utils/adt/datetime.c   | 8 ++++----
src/backend/utils/adt/formatting.c | 6 +++---
src/include/utils/datetime.h       | 8 ++++----
3 files changed, 11 insertions(+), 11 deletions(-)