Correct line_item entity reference for memberships when Separate Payments is true

During testing for another issue Pradeep found that there was inconsistent data stored for line_item.entity_table and line_item.entity_id for memberships. When a) Separate Payments was off, the reference was correctly made to the civicrm_membership record. When b) Separate Payments was on, the reference was incorrectly made to the civicrm_contribution record. (I think there has been a change in 4.5 to introduce behaviour in a) but haven't looked for specific issue, and b) was missed during the QA for that.)

To do: work through the over-complicated code in CRM_Contribute_Form_Contribution_Confirm::postProcess to properly implement b).