[concurrency] Add UnownedJob._runSynchronously(on:) to support running a job synchronously on an executor.

I am going to use this to test that we propagate synchronous accesses into
asynchronous tasks access sets.

To ensure this is not ABI, I underscored/marked this as alwaysEmitIntoClient.

Author: gottesmm

stdlib/public/Concurrency/PartialAsyncTask.swift

@@ -13,11 +13,23 @@
 import Swift
 @_implementationOnly import _SwiftConcurrencyShims
 
+@available(SwiftStdlib 5.5, *)
+@_silgen_name("swift_job_run")
+@usableFromInline
+internal func _swiftJobRun(_ job: UnownedJob,
+                           _ executor: UnownedSerialExecutor) -> ()
+
 /// A job is a unit of scheduleable work.
 @available(SwiftStdlib 5.5, *)
 @frozen
 public struct UnownedJob {
   private var context: Builtin.Job
+
+  @_alwaysEmitIntoClient
+  @inlinable
+  public func _runSynchronously(on executor: UnownedSerialExecutor) {
+      _swiftJobRun(self, executor)
+  }
 }
 
 @available(SwiftStdlib 5.5, *)