Issue Description:

It seems NSData.dataUsingEncoding(_) has a memory leak.

See the following code:

import Foundation

func go() {
    let x = "adsdasf".dataUsingEncoding(NSUTF8StringEncoding)
    print(x)
}

for i in 0..<1000000 {
    go()
}

Run this code in either Swift 2.1 or 2.2 and watch the activity monitor. Memory usage keeps increasing.

If instead of {"adsdasf".dataUsingEncoding(NSUTF8StringEncoding)} one simply does {NSData()} all seems fine.

So this suggests to me that some memory leak is happening in the dataUsingEncoding(_) function.

I'm not entirely sure if this is Swift related (i.e. goed wrong somewhere in the Objective-C <-> Swift bridging) or if it is purely in Foundation, but I thought to start somewhere 🙂.
If it is Foundation related I should probably file a radar?