Fix race conditions in test class used throughout the std::thread tests.

The test class 'G' reads and writes to the same static variables in its
constructor, destructor and call operator. When threads are
constructed using `std::thread t((G()))` there is a race condition between the
destruction of the temporary and the execution of `G::operator()()`.

The fix is to simply create the input before creating the thread.


git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@233946 91177308-0d34-0410-b5e6-96231b3b80d8

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

@@ -9,6 +9,9 @@
 //
 // UNSUPPORTED: libcpp-has-no-threads
 
+// NOTE: TSAN will report this test as leaking a thread.
+// XFAIL: tsan
+
 // <thread>
 
 // class thread
@@ -53,8 +56,11 @@ int main()
     {
         assert(G::n_alive == 0);
         assert(!G::op_run);
-        std::thread t((G()));
-        std::this_thread::sleep_for(std::chrono::milliseconds(250));
+        G g;
+        {
+          std::thread t(g);
+          std::this_thread::sleep_for(std::chrono::milliseconds(250));
+        }
     }
     assert(false);
 }