[C++] Support IS DISTINCT and IS NOT DISTINCT expression.

### Describe the enhancement requested

Ordinary comparison operators yield null (signifying “unknown”), not true or false, when either input is null. For example, 7 = NULL yields null, as does 7 <> NULL. When this behavior is not suitable, use the IS [ NOT ] DISTINCT FROM predicates:

a IS DISTINCT FROM b
a IS NOT DISTINCT FROM b

usage:

```
select * from test where a is distinct from b;
```

or

```
select * from test where a is not distinct from b;
```

reference: https://www.postgresql.org/docs/current/functions-comparison.html

### Component(s)

C++