Fix incorrect Datum conversion in timestamptz_trunc_internal()

Fix incorrect Datum conversion in timestamptz_trunc_internal()

The code used a PG_RETURN_TIMESTAMPTZ() where the return type is
TimestampTz and not a Datum.

On 64-bit systems, there is no effect since this just ends up casting
64-bit integers back and forth.  On 32-bit systems, timestamptz is
pass-by-reference.  PG_RETURN_TIMESTAMPTZ() allocates new memory and
returns the address, meaning that the caller could interpret this as a
timestamp value.

The effect is using "date_trunc(..., 'infinity'::timestamptz) will
return random values (instead of the correct return value 'infinity').

Bug introduced in commit d85ce012f99f.

Author: Peter Eisentraut <peter@eisentraut.org>
Reviewed-by: Tom Lane <tgl@sss.pgh.pa.us>
Discussion: https://postgr.es/m/2d320b6f-b4af-4fbc-9eec-5d0fa15d187b@eisentraut.org
Discussion: https://postgr.es/m/4bf60a84-2862-4a53-acd5-8eddf134a60e@eisentraut.org
Backpatch-through: 18