pgsql: Break out xxx2yyy_opt_overflow APIs for more datetime conversion

Break out xxx2yyy_opt_overflow APIs for more datetime conversions.

Previous commits invented timestamp2timestamptz_opt_overflow,
date2timestamp_opt_overflow, and date2timestamptz_opt_overflow
functions to perform non-error-throwing conversions between
datetime types. This patch completes the set by adding
timestamp2date_opt_overflow, timestamptz2date_opt_overflow,
and timestamptz2timestamp_opt_overflow.

In addition, adjust timestamp2timestamptz_opt_overflow so that it
doesn't throw error if timestamp2tm fails, but treats that as an
overflow case. The situation probably can't arise except with an
invalid timestamp value, and I can't think of a way that that would
happen except data corruption. However, it's pretty silly to have a
function whose entire reason for existence is to not throw errors for
out-of-range inputs nonetheless throw an error for out-of-range input.

The new APIs are not used in this patch, but will be needed in
upcoming btree_gin changes.

Author: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
Reviewed-by: Arseniy Mukhin <arseniy(dot)mukhin(dot)dev(at)gmail(dot)com>
Discussion: https://postgr.es/m/262624.1738460652@sss.pgh.pa.us

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/0059bbe1ecaa5f7f19a8b3aae059f352c02e1d88

Modified Files
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src/backend/utils/adt/date.c | 86 ++++++++++++++++++++++++++++++++++++++-
src/backend/utils/adt/timestamp.c | 81 +++++++++++++++++++++++++++++-------
src/include/utils/date.h | 2 +
src/include/utils/timestamp.h | 3 ++
4 files changed, 156 insertions(+), 16 deletions(-)