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.member/joinable.pass.cpp

@@ -45,7 +45,8 @@ bool G::op_run = false;
 int main()
 {
     {
-        std::thread t0((G()));
+        G g;
+        std::thread t0(g);
         assert(t0.joinable());
         t0.join();
         assert(!t0.joinable());