MySQL 8.0 adds a generated column to the two tables mysql.server_cost
and mysql.engine_cost.  MySQL 5.7 fails if system tables contain
generated columns.  Hence, downgrade to 5.7 will not work when cost
tables contain generated columns from 8.0.

This patch is for 5.7 and backports fix to read_cost_constants() from
8.0.  mysql_system_tables_fix.sql is changed to remove columns
should they exist.  This will make mysql_upgrade, if run during
downgrade, restore these tables to its original form.

sql/opt_costconstantcache.cc
   In order for resolving of generated column to work correctly when
   opening cost tables, lex_start() must be called.

scripts/mysql_system_tables_fix.sql
   Add SQL code to remove the columns from 8.0 should they exist.

scripts/mysql_system_tables.sql
   Specify only non-default values in insert statements for cost tables.
   This way mysql_upgrade does not give errors if extra columns exist.

mysql-test/t/opt_costmodel_downgrade.test
mysql-test/r/opt_costmodel_downgrade.result
   Test to verfiy that 5.7 does not crash should generated columns exist in
   cost tables.  Also verifies that running mysql_upgrade will remove
   the columns.