[Support] Use unique_function for ThreadPool.

[lhames]

Summary

This allows move-only function objects (including lambdas that capture move-only
objects) to be passed to a ThreadPool instance for execution.

[lhames]

This blew up on the builders. The MSVC implementation of packaged_task does not support unique_function (at least at the moment).

llvm/include/llvm/Support/ThreadPool.h

 //===-- llvm/Support/ThreadPool.h - A ThreadPool implementation -*- C++ -*-===//
 //
 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
 // See https://llvm.org/LICENSE.txt for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
 //===----------------------------------------------------------------------===//
 //
 // This file defines a crude C++11 based thread pool.
 //
 //===----------------------------------------------------------------------===//
 
 #ifndef LLVM_SUPPORT_THREAD_POOL_H
 #define LLVM_SUPPORT_THREAD_POOL_H
 
 #include "llvm/ADT/BitVector.h"
+#include "llvm/ADT/FunctionExtras.h"
 #include "llvm/Config/llvm-config.h"
 #include "llvm/Support/Threading.h"
 #include "llvm/Support/thread.h"
 
 #include <future>
 
 #include <atomic>
 #include <condition_variable>
 #include <functional>
 #include <memory>
 #include <mutex>
 #include <queue>
 #include <utility>
 
 namespace llvm {
 
 /// A ThreadPool for asynchronous parallel execution on a defined number of
 /// threads.
 ///
 /// The pool keeps a vector of threads alive, waiting on a condition variable
 /// for some work to become available.
 class ThreadPool {
 public:
-  using TaskTy = std::function<void()>;
+  using TaskTy = unique_function<void()>;
   using PackagedTaskTy = std::packaged_task<void()>;
 
   /// Construct a pool using the hardware strategy \p S for mapping hardware
   /// execution resources (threads, cores, CPUs)
   /// Defaults to using the maximum execution resources in the system, but
   /// accounting for the affinity mask.
   ThreadPool(ThreadPoolStrategy S = hardware_concurrency());