Disallow USING clause when altering type of generated column

Disallow USING clause when altering type of generated column

This does not make sense.  It would write the output of the USING
clause into the converted column, which would violate the generation
expression.  This adds a check to error out if this is specified.

There was a test for this, but that test errored out for a different
reason, so it was not effective.

Reported-by: Jian He <jian.universality@gmail.com>
Reviewed-by: Yugo NAGATA <nagata@sraoss.co.jp>
Discussion: https://www.postgresql.org/message-id/flat/c7083982-69f4-4b14-8315-f9ddb20b9834%40eisentraut.org