Break out xxx2yyy_opt_overflow APIs for more datetime conversions.

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@sss.pgh.pa.us>
Reviewed-by: Arseniy Mukhin <arseniy.mukhin.dev@gmail.com>
Discussion: http://postgr.es/m/262624.1738460652@sss.pgh.pa.us