[Concurrency] Add implementations of run and enqueue for built-in clocks.

The built-in clocks should have implementations of `run` and `enqueue`,
to allow derived clocks to call those implementations.

Author: al45tair

stdlib/public/Concurrency/ContinuousClock.swift

@@ -211,3 +211,36 @@ extension ContinuousClock.Instant: InstantProtocol {
     rhs.duration(to: lhs)
   }
 }
+
+#if !$Embedded
+@available(StdlibDeploymentTarget 6.2, *)
+extension ContinuousClock {
+
+  public func run(_ job: consuming ExecutorJob,
+                  at instant: Instant,
+                  tolerance: Duration?) {
+    guard let executor = Task<Never,Never>.currentSchedulingExecutor else {
+      fatalError("no scheduling executor is available")
+    }
+
+    executor.enqueue(job, at: instant,
+                     tolerance: tolerance,
+                     clock: self)
+  }
+
+  public func enqueue(_ job: consuming ExecutorJob,
+                      on executor: some Executor,
+                      at instant: Instant,
+                      tolerance: Duration?) {
+    if let schedulingExecutor = executor.asSchedulingExecutor {
+      schedulingExecutor.enqueue(job, at: instant,
+                                 tolerance: tolerance,
+                                 clock: self)
+    } else {
+      let trampoline = job.createTrampoline(to: executor)
+      run(trampoline, at: instant, tolerance: tolerance)
+    }
+  }
+
+}
+#endif

stdlib/public/Concurrency/SuspendingClock.swift

@@ -189,3 +189,36 @@ extension SuspendingClock.Instant: InstantProtocol {
     rhs.duration(to: lhs)
   }
 }
+
+#if !$Embedded
+@available(StdlibDeploymentTarget 6.2, *)
+extension SuspendingClock {
+
+  public func run(_ job: consuming ExecutorJob,
+                  at instant: Instant,
+                  tolerance: Duration?) {
+    guard let executor = Task<Never,Never>.currentSchedulingExecutor else {
+      fatalError("no scheduling executor is available")
+    }
+
+    executor.enqueue(job, at: instant,
+                     tolerance: tolerance,
+                     clock: self)
+  }
+
+  public func enqueue(_ job: consuming ExecutorJob,
+                      on executor: some Executor,
+                      at instant: Instant,
+                      tolerance: Duration?) {
+    if let schedulingExecutor = executor.asSchedulingExecutor {
+      schedulingExecutor.enqueue(job, at: instant,
+                                 tolerance: tolerance,
+                                 clock: self)
+    } else {
+      let trampoline = job.createTrampoline(to: executor)
+      run(trampoline, at: instant, tolerance: tolerance)
+    }
+  }
+
+}
+#endif