Fixed #24171 -- Fixed failure with complex aggregate query and expressions

The query used a construct of qs.annotate().values().aggregate() where
the first annotate used an F-object reference and the values() and
aggregate() calls referenced that F-object.

Also made sure the inner query's select clause is as simple as possible,
and made sure .values().distinct().aggreate() works correctly.

tests/expressions/models.py

@@ -12,6 +12,7 @@ from django.utils.encoding import python_2_unicode_compatible
 class Employee(models.Model):
     firstname = models.CharField(max_length=50)
     lastname = models.CharField(max_length=50)
+    salary = models.IntegerField(blank=True, null=True)
 
     def __str__(self):
         return '%s %s' % (self.firstname, self.lastname)