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.
#include <gtest/gtest.h>
#include <Storages/transformQueryForExternalDatabase.h>
#include <Parsers/ParserSelectQuery.h>
#include <Parsers/parseQuery.h>
#include <DataTypes/DataTypesNumber.h>
#include <DataTypes/DataTypeDateTime.h>
#include <DataTypes/DataTypeString.h>
#include <Interpreters/Context.h>
#include <Interpreters/TreeRewriter.h>
#include <Databases/DatabaseMemory.h>
#include <Storages/StorageMemory.h>
#include <Common/tests/gtest_global_context.h>
#include <Common/tests/gtest_global_register.h>
using namespace DB;
struct State
{
    State(const State&) = delete;