Open Radar

 
DispatchTime.uptimeNanoseconds appears to be returning the dispatch_time_t value returned from DispatchTime.rawValue. dispatch_time_t is not necessarily measured in nanoseconds. This causes statements such as

```
DispatchTime(uptimeNanoseconds: DispatchTime.now().uptimeNanoseconds + offset)
```

to return unexpected values.

Steps to Reproduce:
Run the following test both in an iOS simulator and on an iOS device:

```
        let seconds = 1
        let now = DispatchTime.now()
        let oneSecondFromNow = now + .seconds(seconds)
        print("current dispatch time: \(now)")
        print("rawValue measured: \(oneSecondFromNow.rawValue - now.rawValue) dispatch_time_t in \(seconds) seconds")
        print("uptimeNanoseconds measured: \(oneSecondFromNow.uptimeNanoseconds - now.uptimeNanoseconds) nanoseconds in \(seconds) seconds")
        print("NSEC_PER_SEC = \(NSEC_PER_SEC)")
        XCTAssertEqual(oneSecondFromNow.uptimeNanoseconds, now.uptimeNanoseconds + NSEC_PER_SEC)
```

Expected Results:
As seen on an iPhone 6 simulator:

current dispatch time: DispatchTime(rawValue: 125467249779612)
rawValue measured: 1000000000 dispatch_time_t in 1 seconds
uptimeNanoseconds measured: 1000000000 nanoseconds in 1 seconds
NSEC_PER_SEC = 1000000000

Test passes since oneSecondFromNow.uptimeNanoseconds == 125468249779612 == now.uptimeNanoseconds + NSEC_PER_SEC

Actual Results:
Seen on an iPhone 6:

current dispatch time: DispatchTime(rawValue: 4710219239612)
rawValue measured: 24000000 dispatch_time_t in 1 seconds
uptimeNanoseconds measured: 24000000 nanoseconds in 1 seconds
NSEC_PER_SEC = 1000000000
XCTAssertEqual failed: ("4710243239612") is not equal to ("4711219239612")