[stdlib] add single-element assign to UnsafeMutablePointer
[stdlib] add single-element assign to UnsafeMutablePointer

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