Revert "[Support] Use unique_function rather than std::function for ThreadPool TaskTy."

This reverts commit d9c8b0256c.

Some MSVC std::packaged_task implementations are not compatible with move-only types.
This caused failures on some of the Windows builders (e.g.
http://lab.llvm.org:8011/builders/sanitizer-windows/builds/69412).

Reverting until I can come up with a workaround.

llvm/include/llvm/Support/ThreadPool.h

@@ -13,7 +13,6 @@
 #ifndef LLVM_SUPPORT_THREAD_POOL_H
 #define LLVM_SUPPORT_THREAD_POOL_H
 
-#include "llvm/ADT/FunctionExtras.h"
 #include "llvm/Config/llvm-config.h"
 #include "llvm/Support/Threading.h"
 #include "llvm/Support/thread.h"
@@ -37,7 +36,7 @@ namespace llvm {
 /// for some work to become available.
 class ThreadPool {
 public:
-  using TaskTy = unique_function<void()>;
+  using TaskTy = std::function<void()>;
   using PackagedTaskTy = std::packaged_task<void()>;
 
   /// Construct a pool using the hardware strategy \p S for mapping hardware

llvm/unittests/Support/ThreadPool.cpp

@@ -133,13 +133,6 @@ TEST_F(ThreadPoolTest, Async) {
   ASSERT_EQ(2, i.load());
 }
 
-TEST_F(ThreadPoolTest, NonCopyableTask) {
-  CHECK_UNSUPPORTED();
-  ThreadPool Pool;
-  Pool.async([P = std::make_unique<int>()] {});
-  Pool.wait();
-};
-
 TEST_F(ThreadPoolTest, GetFuture) {
   CHECK_UNSUPPORTED();
   ThreadPool Pool(hardware_concurrency(2));