Expand virtual generated columns for ALTER COLUMN TYPE

Expand virtual generated columns for ALTER COLUMN TYPE

For the subcommand ALTER COLUMN TYPE of the ALTER TABLE command, the
USING expression may reference virtual generated columns.  These
columns must be expanded before the expression is fed through
expression_planner and the expression-execution machinery.  Failing to
do so can result in incorrect rewrite decisions, and can also lead to
"ERROR:  unexpected virtual generated column reference".

Reported-by: Alexander Lakhin <exclusion@gmail.com>
Reviewed-by: jian he <jian.universality@gmail.com>
Discussion: https://postgr.es/m/b5f96b24-ccac-47fd-9e20-14681b894f36@gmail.com