should check collations when creating partitioned index

When creating a partitioned index, the partition key must be a subset of
the index's columns. DefineIndex() explains:

* If this table is partitioned and we're creating a unique index,
primary
* key, or exclusion constraint, make sure that the partition key is a
* subset of the index's columns. Otherwise it would be possible to
* violate uniqueness by putting values that ought to be unique in
* different partitions.

But this currently doesn't check that the collations between the
partition key and the index definition match. So you can construct a
unique index that fails to enforce uniqueness.

Here is a non-partitioned case for reference:

create collation case_insensitive (provider=icu,
locale='und-u-ks-level2', deterministic=false);
create table t0 (a int, b text);
create unique index i0 on t0 (b collate case_insensitive);
insert into t0 values (1, 'a'), (2, 'A'); -- violates unique constraint

Here is a partitioned case that doesn't work correctly:

create table t1 (a int, b text) partition by hash (b);
create table t1a partition of t1 for values with (modulus 2, remainder 0);
create table t1b partition of t1 for values with (modulus 2, remainder 1);
create unique index i1 on t1 (b collate case_insensitive);
insert into t1 values (1, 'a'), (2, 'A'); -- this succeeds

The attached patch adds the required collation check. In the example,
it would not allow the index i1 to be created.