Add support for joining a subquery in hql

[maca88]

Basic hql syntax:

from Order o 
inner join (
  from Order where OrderId = 10248
) o2 on (o.OrderId - 1) = o2.OrderId

In case the subquery returns multiple values, each value can be referenced by using an alias:

select o, o2.ord, o2.cu
from Order o 
inner join (
  select o1 as ord, o1.Customer as cu
  from Order o1 where o1.OrderId = 10248
) o2 on (o.OrderId - 1) = o2.ord.OrderId

Aliases can be nested in case of nested subqueries:

select 
  o4.ord3, o4.ord3.ShippedTo, o4.cu3, o4.ord3.ShippingAddress.City,
  o4.sub2.ord2, o4.sub2.ord2.ShippedTo, o4.sub2.cu2, o4.sub2.cu2.Address.City,
  o4.sub2.sub1.ord1, o4.sub2.sub1.ord1.ShippedTo, o4.sub2.sub1.cu1, o4.sub2.sub1.cu1.Address.City
from Order o5
inner join (
  select o as ord3, o.Customer as cu3, o3 as sub2
  from Order o
  left join (
    select o1 as ord2, o1.Customer as cu2, o2 as sub1
    from Order o1
    left join (
      select o0 as ord1, o0.Customer as cu1
      from Order o0
    ) o2 on o1.OrderId = o2.ord1.OrderId-1
  ) o3 on o.OrderId = o3.ord2.OrderId-1
  where o.OrderId = 10248
) o4 on o5.OrderId = o4.ord3.OrderId

This feature is a requirement in order to support the Include method on all databases when paging results. Sql IN operator does not work with composite keys and in some databases even for normal keys (e.g. MySql), so with this feature we can write the following query:

select o
from CompositeOrder o 
inner join (
  select id
  from CompositeOrder
  order by OrderId skip 2 take 2
) o2 on o.id = o2.id
left join fetch o.OrderLines

Known limitation:

• Selecting a hql constructor inside a subquery is not supported

Also fixes #2092

[maca88]

Now subclassPropertyAliases contains also component properties which are needed in order to select component properties from a subquery.

[bahusoid]

And your alias related changes seem to fix issue reported here:
#2092

I see no duplicates in generated SQL

[maca88]

It's not possible to add association join dependent on subquery join

With the last commit the issue is fixed.

[fredericDelaporte]

I have not reviewed the SelectClause changes. I lack knowledge of how it works there.

[fredericDelaporte]

@maca88, this causes #3489 regression.
The query is

insert into Enrolment (Course, Student)
  select e.Course, e.Student from Enrolment e

At the second column, the from is already added, causing the execution to go into the else and not rendering the second column.

In debug, forcing the execution to jump directly to RenderNonScalarIdentifiers (so, not adding again the from to combinedFromElements) allows the test to succeed.

But surely the fix is not as simple as throwing away the else in all cases with something like:

if (processedElements.Add(fromElement))
{
	combinedFromElements.Add(fromElement);
}
RenderNonScalarIdentifiers(fromElement, inheritedExpressions.ContainsKey(e) ? null : e, appender);

[fredericDelaporte]

Doing that change seems to break only HqlDuplicateEntitySelectionSubQuery which looks like testing invalid cases, containing duplicated selection of entities. Why should we support theses cases?