Merge two copies of tuple-building code in pltcl.c.

Merge two copies of tuple-building code in pltcl.c.

Make pltcl_trigger_handler() construct modified tuples using
pltcl_build_tuple_result(), rather than its own copy of essentially
the same logic.  This results in slightly different message wording for
the error cases, and in one case a different SQLSTATE, but it seems
unlikely that any existing applications are depending on any of those
details.

While at it, fix a typo in commit 26abb50c4: pltcl_build_tuple_result was
applying encoding conversion in the wrong direction.  That would be a
back-patchable bug fix, except the code hasn't shipped yet.

Jim Nasby, reviewed by me

Discussion: https://postgr.es/m/d2c6425a-d9e0-f034-f774-4a872c234d89@BlueTreble.com