Analysis:
========
A foreign key constraint cannot reference a secondary index defined
on a generated virtual column. While adding new index/drop existing
column, server internally drops the internal foreign key index and
it leads to choose the virtual secondary index as foreign key index.
But innodb doesn't allow foreign key constraint reference to
secondary virtual index.

Fix:
===
Allow foreign key constraint refer to secondary index defined on
a generated virutal column.

Reviewed-by: Jimmy Yang<jimmy.yang@oracle.com>
RB: 13586