AD Armor VS MR Armor

As a person who likes to play tank, armor seems very useless vs ADCs compared to magic resist vs APCs. I was fed, building Ninja Tab, Randuins, Thornmail, and this twitch decided he could just tear me down in like 4 autos with his ult. I'm confused ... Is there a hidden secret about building tank? Sometimes I feel like as a Tank, I'd be more useful building damage and try to kill the carry faster than they can kill me. Now when it comes to a heavy MR team, I do quite well building some nice MR items like abyssal mask, spirit visage, etc. Summary: I think we need a tank item like Adaptive Helm (edit, thanks to commenter below) that reduces damage from ad the more you get hit, or just some new tank item that can avoid tanks actually getting nailed in the behind all the time.
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