planner: warn if manual hints are overwritten by bindings

Enhancement

In the example below, the user wants to use a hint to force the optimizer to use index b, but it's still using index a, since a binding overwrites the hint. We should warn in this scenario to avoid confusing users.

mysql> create table t (a int, b int, key(a), key(b));
Query OK, 0 rows affected (0.016 sec)

mysql> create global binding using select /*+ use_index(t, a) */ * from t where a=1 and b=1;                                                                                                                                   
Query OK, 0 rows affected (0.006 sec)

mysql> explain select /*+ use_index(t, b) */ * from t where a=1 and b=1;
+-------------------------------+---------+-----------+---------------------+---------------------------------------------+
| id                            | estRows | task      | access object       | operator info                               |
+-------------------------------+---------+-----------+---------------------+---------------------------------------------+
| IndexLookUp_8                 | 0.01    | root      |                     |                                             |
| ├─IndexRangeScan_5(Build)     | 10.00   | cop[tikv] | table:t, index:a(a) | range:[1,1], keep order:false, stats:pseudo |
| └─Selection_7(Probe)          | 0.01    | cop[tikv] |                     | eq(test.t.b, 1)                             |
|   └─TableRowIDScan_6          | 10.00   | cop[tikv] | table:t             | keep order:false, stats:pseudo              |
+-------------------------------+---------+-----------+---------------------+---------------------------------------------+