This is to mark that a field is indirectly part of a key, which simplifes
checking if we need to have this field up to date to evaluate a key.

For example:
CREATE TABLE t1 (a int, b int as (a) virtual,
                 c int as (b) virtual, index(c))
would mark a and b with PART_INDIRECT_KEY_FLAG.
c is marked with PART_KEY_FLAG as before.