[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.condition/thread.condition.condvar/notify_one.pass.cpp

@@ -20,12 +20,13 @@
 #include <thread>
 #include <cassert>
 
+#include "test_atomic.h"
+
 std::condition_variable cv;
 std::mutex mut;
 
-int test0 = 0;
-int test1 = 0;
-int test2 = 0;
+AtomicInt test1(0);
+AtomicInt test2(0);
 
 void f1()
 {
@@ -64,11 +65,13 @@ int main()
     }
     if (test1 == 2)
     {
+        assert(test2 == 1);
         t1.join();
         test1 = 0;
     }
     else if (test2 == 2)
     {
+        assert(test1 == 1);
         t2.join();
         test2 = 0;
     }
@@ -81,11 +84,13 @@ int main()
     }
     if (test1 == 2)
     {
+        assert(test2 == 0);
         t1.join();
         test1 = 0;
     }
     else if (test2 == 2)
     {
+        assert(test1 == 0);
         t2.join();
         test2 = 0;
     }