Global index supports non unique index.

[Defined2014]

Feature Request

Is your feature request related to a problem? Please describe:

After #45133, we already supported global index with unique index. In order to meet the needs of more users, we plan to further relax the restrictions and allow non-unique indexes to be used as global indexes.

Describe the feature you'd like:

Supports some SQLs below

create table t(a int, b int, index idx(b) global) partition by hash(a);

create table t(a int, b int) partition by hash(a); 
alter table t add index idx(b) global;

Describe alternatives you've considered:

None

Teachability, Documentation, Adoption, Migration Strategy:

Remove the limitaions in https://docs.pingcap.com/tidb/dev/partitioned-table#limitations-of-global-indexes after this issue fixed.