When a table has no PRIMARY KEY, but there is a UNIQUE INDEX
defined on NOT NULL columns that are not column prefixes,
that unique index must be treated as the primary key.

This property was being violated by InnoDB when a column was changed
to NOT NULL, such that a UNIQUE INDEX on that column became eligible
to being treated as a primary key.

innobase_create_key_defs(): Instead of checking each ADD [UNIQUE] INDEX
request, check if a GEN_CLUST_INDEX can be replaced with any unique index
in the altered_table definition. So, we can have new_primary even
if n_add==0.

prepare_inplace_alter_table_dict(): When the table is not being rebuilt,
assert that TABLE_SHARE::primary_key is not changing.

RB: 13595
Reviewed-by: Kevin Lewis <kevin.lewis@oracle.com>