When using a foreign table (e.g. a mysql table) clickhouse doesn't seem to properly send the result of functions if they are called inside `WHERE` statements.

Executing `SELECT foo FROM foreign_table WHERE a=10 AND b='2019-10-05'` will send a statement like:

```
SELECT `foo`, `a`, `b` FROM `db`.`foreign_table`  WHERE (`a` = 10) AND (`b` = '2019-10-05')
```

But executing `SELECT foo FROM foreign_table WHERE a=10 AND b=toDate('2019-10-05')` will send:

```
SELECT `foo`, `a`, `b` FROM `db`.`foreign_table`  WHERE (`a` = 10)
```

I assume that the function is executed afterwards and `b` is parsed clickhouse-side.

I'm not sure if there is an easy fix for this, but I assume this would help with the efficiency of mysql foreign tables and might be critical in some high-load scenarios where one wants to filter on indexed columns.

My specific issue here is different and relates to the clickhouse mindsdb integration, but I think this "bug" (if you'd call it such) is more generic.
{
    const State & state = State::instance();
    check(state, 1,
          "SELECT field FROM table WHERE field IS NULL",
          R"(SELECT "field" FROM "test"."table" WHERE "field" IS NULL)");
    check(state, 1,
          "SELECT field FROM table WHERE field IS NOT NULL",
          R"(SELECT "field" FROM "test"."table" WHERE "field" IS NOT NULL)");
    check(state, 1,
          "SELECT field FROM table WHERE isNull(field)",
          R"(SELECT "field" FROM "test"."table" WHERE "field" IS NULL)");
    check(state, 1,
          "SELECT field FROM table WHERE isNotNull(field)",
          R"(SELECT "field" FROM "test"."table" WHERE "field" IS NOT NULL)");
}