DESCRIPTION:
------------
During upgrade of partitioned tables from 5.7 to 8.0, when a
prefix key is used by the partitioning function, the prefix
length is ignored, and the full column length is considered
instead. Hence, the table might incorrectly be rejected from
being upgraded because its partition field length is found
to be too large, e.g. for a table like the following:

CREATE TABLE t1(a VARCHAR(10000),
                b VARCHAR(10),
                PRIMARY KEY(a(100), b)
) PARTITION BY KEY() PARTITIONS 2;

ANALYSIS:
---------
When creating/opening/upgrading a table, the
unpack_partition_info() function reads the length of the
partitioned fields. There is a difference in behaviour when
a table is created (or opened from the DD for the first
time) in 8.0, and when a table is upgraded from 5.7, which
is detailed below.

During CREATE/OPEN TABLE:
    open_table_from_share() is called, which
    - Creates a copy of the key_info from TABLE_SHARE object
    to TABLE object.
    - Allocates a new Field object, with the field_length
    (eg: originally 10000) set to the key_part length (which
    is the length of the prefix key, eg: 100)
    - Points key_part->field to the new Field.

    Hence, unpack_partition_info()
    - Gets the TABLE object with the Field pointer having
    the correct length of partitioned fields.

However, during UPGRADE from 5.7 to 8.0:
    open_table_from_share() is never called, instead,
    migrate_table_to_dd() is called, which simply
    - Points the key_info of TABLE to that of TABLE_SHARE.

    Hence, unpack_partition_info()
    - Gets the TABLE object with the Field pointer having
    the unmodified field_length (eg: 10000)
    - Causes the upgrade to be wrongfully rejected (because
    partition field length is found to be too large).

FIX:
----
Since open_table_from_share() performs the same tasks as
this fix requires, we move the relevant code into a new
function called create_key_part_field_with_prefix_length().

This new function is then called in migrate_table_to_dd()
during UPGRADE. This ensures that unpack_partition_info()
reads the correct prefix length of partitioned fields, and
upgrade does not fail in this scenario.

Change-Id: I8ead62149b19ddc1d4546a34676c8e14369a5493