This build isn´t to make morgana a support character, but to make her deal massive damage with her skills.

The choice for finish mejai´s soulstealer before finish the boots are because she is a good ganker with her first skill,so she can gank at level 3 or ealier if you want.
Rilays take care of her hp,and make her tormented soil cause slow in area.
Rabadon grown up her AP, at that time in the game, mejai´s need at least 7 charges.
Zhonya Hourglass make you a good bait, with her shield, you can go in the middle of the battle,take damage,but don´t die.
The void staff isnt the only choice for last item, but it make her magic damage realy grow up, besides,i like lich bane for her too.

This skill sequence is to make morgana kill and make assists, if you are under pressure, pick the shield at low levels.