[5.5] Add support to use subqueries as a where condition on a join clause

[rodrigopedra]

As reported on issue #19695 when a user tries to use a ->whereIn() or a ->whereExists() with a sub-query in a join clause, the query builder does not build the inner query where condition as expected.

Example:

<?php

\Route::get( '/test', function () {
    $builder = \DB::table( 'users' )
        ->join( 'password_resets', function ( JoinClause $join ) {
            $join->on( 'users.email', '=', 'password_resets.email' );
            $join->whereIn( 'email', function ( $query ) {
                // dummy example just to demonstrate issue
                $query->select( 'email' )
                    ->from( 'users' )
                    ->where( 'id', '>', 1 );
            } );
        } );

    return $builder->toSql();
} );

Generates this SQL statment:

select * 
from 
    "users" 
    inner join "password_resets" 
        on "users"."email" = "password_resets"."email" 
        and "email" in (select "email" from "users" on "id" > ?)

instead of the expected one:

select * 
from 
    "users" 
    inner join "password_resets" 
        on "users"."email" = "password_resets"."email" 
        and "email" in (select "email" from "users" WHERE "id" > ?)

The is caused due to JoinClause@newQuery() method override returns a new JoinClause instance instead of a Query Builder instance. This makes Grammar@concatenateWhereClauses() to compile the nested query where clause not as expected.

I tried to change JoinClause@newQuery() method override to return a Query Builder instance. But doing so fails the nested where tests within a join clause.

This PR adds a protected method on the Query Builder that returns a new Builder instance for sub-queries so that the JoinClause class can override it properly.

Tests were added to cover these additional cases.

[themsaid]

I believe this is ok.