Optimizer support for functional indexes: handle duplicate columns

See MDEV-35615, section Handle Duplicate virtual columns.

Note: this task will get more important when MDEV-35853 is done.
Currently, one has to add virtual columns manually before adding and index:

ALTER TABLE t1

  ADD vcol  $vcol_type AS ($vcol_expr),

  ADD INDEX( .. .vcol ...); 

so, they can as well make sure they do not create multiple virtual columns with the same definition.

After MDEV-35853, one would be able to just do

ALTER TABLE t1 

  ADD INDEX idx1 ( $expression),

  ADD INDEX idx2 (... $expression ...);

Here, the "default" action is to create two virtual columns for use in idx1 and idx2. Which would prevent one of them from being used. And so, would require this MDEV to be implemented.