Apparently building armor doesn't help at all. Black Cleaver, Last Whisper and lethality - and you are still getting exploded in a split second. And I've been on both sides: when I play ad champs, I feel impactful regardless of what enemy teams build.
build armor
have things that reduce their damage... if they are on hit attack speed base items like 
i play a tank main top lane and the all ad comp is very common and i build pure armor and do just fine.
http://matchhistory.na.leagueoflegends.com/en/#match-details/NA1/2485414650/203807166?tab=overview
^^^^^^^^^^^^
observe this leet build....
but on a serious note all you need is
tabis/thornmail
and
raw armor, as much as you can get. doesn't matter which ones.
If you are a tank, build a mix of hp and armor such as that you have the most effective hp against phyisical damage after the armor reduction/ignore.
