Refactor: separate function to find all objects depending on a column

Move code from ATExecAlterColumnType() that finds the all the objects
that depend on the column to a separate function.  A future patch will
reuse this code.

Author: Amul Sul <sulamul@gmail.com>
Discussion: https://www.postgresql.org/message-id/flat/CAAJ_b94yyJeGA-5M951_Lr+KfZokOp-2kXicpmEhi5FXhBeTog@mail.gmail.com

Author: petere