So did Riot intend for Yi and Fiora to build tank?

They don't really build tank, they build bruiser. It's a massive problem with fiora because with only two damage items her passive does like 20% max health true damage or something. It's less of a problem on yi because his damage potential actually falls off as he builds tank, it's still extremely noticeable with his E active but it's not overbearing considering how he has no CC, his only escape is tied to his main dueling tool, and is still pretty squishy (unless you're a god with alpha and W timings, which is why cowsep's yi is feared so much).

I think the only person I have seen build tank Yi is Cowsep

[zombie-brand-mindblown]

Yeah, build squishy and get 1shotted/ kited by the enemy ranged champs. Sounds great.