Don't reorder PrimaryKeyConstraint columns if explicit

Don't reorder PrimaryKeyConstraint columns if explicit

Dialed back the "order the primary key columns per auto-increment"
described in :ref:`change_mysql_3216` a bit, so that if the
:class:`.PrimaryKeyConstraint` is explicitly defined, the order
of columns is maintained exactly, allowing control of this behavior
when necessary.

Change-Id: I9e7902c57a96c15968a6abf53e319acf15680da0
Fixes: #3726