[stdlib] simplify `update(fromContentsOf:)`
[stdlib] simplify `update(fromContentsOf:)`

- one fewer variable, one fewer return point

roambotics/swift,glessard/swift,apple/swift,roambotics/swift,roambotics/swift,roambotics/swift,glessard/swift,apple/swift,glessard/swift,apple/swift,glessard/swift,apple/swift,roambotics/swift,apple/swift,apple/swift,glessard/swift,roambotics/swift,glessard/swift