Disallow USING clause when altering type of generated column

[Peter Eisentraut]

A USING clause when altering the type of a generated column does not 
make sense.  It would write the output of the USING clause into the 
converted column, which would violate the generation  expression.

This patch 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.

discovered by Jian He at [0]

[0]: 
https://www.postgresql.org/message-id/cacjufxegpytfe79hbsmeoboivfnnprsw7gjvk67m1x2mqgg...@mail.gmail.com