pgsql: Reject opclass options in ON CONFLICT clause

Reject opclass options in ON CONFLICT clause

It's as pointless as ASC/DESC and NULLS FIRST/LAST are, so reject all of
them in the same way. While at it, normalize the others' error messages
to have less translatable strings. Add tests for these errors.

Noticed while reviewing recent INSERT ON CONFLICT patches.

Author: Álvaro Herrera <alvherre(at)kurilemu(dot)de>
Reviewed-by: Peter Geoghegan <pg(at)bowt(dot)ie>
Discussion: https://postgr.es/m/202511271516.oiefpvn3z27m@alvherre.pgsql

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/630a93799d538c35c94187e07ef64d566a573a4e

Modified Files
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src/backend/parser/parse_clause.c | 22 +++++++++++++++-------
src/test/regress/expected/insert_conflict.out | 13 +++++++++++++
src/test/regress/sql/insert_conflict.sql | 5 +++++
3 files changed, 33 insertions(+), 7 deletions(-)