AST: Minor optimization for minimal composition computation
AST: Minor optimization for minimal composition computation

Trust the `GenericSignature` class with upholding its invariants and assert
that a superclass bound is in the right place instead of prepending it.

ahoppen/swift,roambotics/swift,glessard/swift,apple/swift,atrick/swift,benlangmuir/swift,benlangmuir/swift,ahoppen/swift,apple/swift,ahoppen/swift,apple/swift,apple/swift,roambotics/swift,benlangmuir/swift,ahoppen/swift,glessard/swift,benlangmuir/swift,atrick/swift,glessard/swift,roambotics/swift,glessard/swift,atrick/swift,glessard/swift,glessard/swift,JGiola/swift,JGiola/swift,atrick/swift,apple/swift,roambotics/swift,roambotics/swift,atrick/swift,benlangmuir/swift,roambotics/swift,atrick/swift,ahoppen/swift,JGiola/swift,JGiola/swift,benlangmuir/swift,JGiola/swift,ahoppen/swift,JGiola/swift,apple/swift