Dispatch: correct DispatchTimeInterval calculation on ASi

There is no guarantee that Mach ticks and nanoseconds have a 1:1
relationship.  Use the `uptimeNanoseconds` field which will convert the
raw value (which is in Mach ticks) to the time unit of nanoseconds.
This was caught by the Dispatch stridable test in the Swift test suite.

rdar://problem/56623421

Author: compnerd

stdlib/public/Darwin/Dispatch/Schedulers+DispatchQueue.swift

@@ -45,8 +45,8 @@ extension DispatchTime /* : Strideable */ {
     typealias Stride = DispatchTimeInterval
 
     public func distance(to other: DispatchTime) -> DispatchTimeInterval {
-        let lhs = other.rawValue
-        let rhs = rawValue
+        let lhs = other.uptimeNanoseconds
+        let rhs = uptimeNanoseconds
         if lhs >= rhs {
             return DispatchTimeInterval.nanoseconds(Int(lhs - rhs))
         } else {