[libcxx] Add Atomic test helper and fix TSAN failures.

Summary:
This patch attempts to fix the last 3 TSAN failures on the libc++ bot (http://lab.llvm.org:8011/builders/libcxx-libcxxabi-x86_64-linux-ubuntu-tsan/builds/143). This patch also adds a `Atomic` test type that can be used where `<atomic>` cannot.

`wait.exception.pass.cpp` and `wait_for.exception.pass.cpp` were failing because the test replaced `std::terminate` with `std::exit`. `std::exit` would asynchronously run the TLS and static destructors and this would cause a race condition. See PR22606 and D8802 for more details. 

This is fixed by using `_Exit` to prevent cleanup.

`notify_all_at_thread_exit.pass.cpp` exercises the same race condition but for different reasons. I fixed this test by manually joining the thread before beginning program termination.

Reviewers: EricWF, mclow.lists

Subscribers: cfe-commits

Differential Revision: http://reviews.llvm.org/D11046

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@245389 91177308-0d34-0410-b5e6-96231b3b80d8

test/std/thread/thread.threads/thread.thread.class/thread.thread.constr/F.pass.cpp

@@ -22,6 +22,8 @@
 #include <cstdlib>
 #include <cassert>
 
+#include "test_macros.h"
+
 unsigned throw_one = 0xFFFF;
 
 void* operator new(std::size_t s) throw(std::bad_alloc)
@@ -75,7 +77,7 @@ public:
 int G::n_alive = 0;
 bool G::op_run = false;
 
-#ifndef _LIBCPP_HAS_NO_VARIADICS
+#if TEST_STD_VER >= 11
 
 class MoveOnly
 {
@@ -137,7 +139,7 @@ int main()
             assert(!G::op_run);
         }
     }
-#ifndef _LIBCPP_HAS_NO_VARIADICS
+#if TEST_STD_VER >= 11
     {
         assert(G::n_alive == 0);
         assert(!G::op_run);
@@ -150,5 +152,5 @@ int main()
         std::thread t = std::thread(MoveOnly(), MoveOnly());
         t.join();
     }
-#endif  // _LIBCPP_HAS_NO_VARIADICS
+#endif
 }

test/std/thread/thread.threads/thread.thread.class/thread.thread.destr/dtor.pass.cpp

@@ -9,8 +9,6 @@
 //
 // UNSUPPORTED: libcpp-has-no-threads
 
-// NOTE: TSAN will report this test as leaking a thread.
-// XFAIL: tsan
 
 // <thread>
 
@@ -47,7 +45,7 @@ bool G::op_run = false;
 
 void f1()
 {
-    std::exit(0);
+    std::_Exit(0);
 }
 
 int main()