GSB: Assert that a minimal signature doesn't have protocols refined by other protocols
GSB: Assert that a minimal signature doesn't have protocols refined by other protocols

I'm looking at a bug where we end with a signature like

    protocol B { ... }
    protocol BB : B { }

    <Self where Self : B, Self : BB>

While this one be a one-off bug, it's easy enough to check for this
condition with an assert here.

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