Fix the DispatchTime.init(uptimeNanoseconds:) initializer to convert its argument to Mach absolute time instead of treating it as Mach absolute time.

(Radar 29660448)

Author: ktopley-apple

stdlib/public/SDK/Dispatch/Time.swift

@@ -1,3 +1,4 @@
+
 //===----------------------------------------------------------------------===//
 //
 // This source file is part of the Swift.org open source project
@@ -41,12 +42,22 @@ public struct DispatchTime : Comparable {
 	/// - Returns: A new `DispatchTime`
 	/// - Discussion: This clock is the same as the value returned by
 	///               `mach_absolute_time` when converted into nanoseconds.
+	///               On some platforms, the nanosecond value is rounded up to a
+	///               multiple of the Mach timebase, using the conversion factors
+	///               returned by `mach_timebase_info()`. The nanosecond equivalent
+	///               of the rounded result can be obtained by reading the
+	///               `uptimeNanoseconds` property.
 	///               Note that `DispatchTime(uptimeNanoseconds: 0)` is
 	///               equivalent to `DispatchTime.now()`, that is, its value
 	///               represents the number of nanoseconds since boot (excluding
 	///               system sleep time), not zero nanoseconds since boot.
 	public init(uptimeNanoseconds: UInt64) {
-		self.rawValue = dispatch_time_t(uptimeNanoseconds)
+		var rawValue = uptimeNanoseconds
+		if (DispatchTime.timebaseInfo.numer != DispatchTime.timebaseInfo.denom) {
+			rawValue = (rawValue * UInt64(DispatchTime.timebaseInfo.denom) 
+				+ UInt64(DispatchTime.timebaseInfo.numer - 1)) / UInt64(DispatchTime.timebaseInfo.numer)
+		}
+		self.rawValue = dispatch_time_t(rawValue)
 	}
 
 	public var uptimeNanoseconds: UInt64 {