GSB: Use a SmallSetVector to store the explicit requirements
GSB: Use a SmallSetVector to store the explicit requirements

We can literally have duplicate requirements (same source and RHS)
when abstract signatures are built from scratch. They're not
flagged as "redundant" since there's no way to distinguish them
in the requirement graph, so just filter them out entirely.

roambotics/swift,roambotics/swift,tkremenek/swift,gregomni/swift,parkera/swift,roambotics/swift,gregomni/swift,xwu/swift,parkera/swift,JGiola/swift,apple/swift,hooman/swift,parkera/swift,parkera/swift,hooman/swift,hooman/swift,gregomni/swift,rudkx/swift,parkera/swift,ahoppen/swift,glessard/swift,xwu/swift,atrick/swift,benlangmuir/swift,rudkx/swift,glessard/swift,atrick/swift,rudkx/swift,apple/swift,gregomni/swift,glessard/swift,hooman/swift,glessard/swift,tkremenek/swift,xwu/swift,ahoppen/swift,xwu/swift,parkera/swift,xwu/swift,atrick/swift,roambotics/swift,benlangmuir/swift,glessard/swift,xwu/swift,xwu/swift,benlangmuir/swift,tkremenek/swift,gregomni/swift,apple/swift,rudkx/swift,JGiola/swift,tkremenek/swift,atrick/swift,ahoppen/swift,benlangmuir/swift,rudkx/swift,atrick/swift,ahoppen/swift,atrick/swift,ahoppen/swift,parkera/swift,parkera/swift,rudkx/swift,benlangmuir/swift,roambotics/swift,apple/swift,JGiola/swift,apple/swift,hooman/swift,hooman/swift,gregomni/swift,tkremenek/swift,ahoppen/swift,roambotics/swift,hooman/swift,tkremenek/swift,JGiola/swift,JGiola/swift,JGiola/swift,benlangmuir/swift,glessard/swift,tkremenek/swift,apple/swift