[libcxx] [test] Fix spurious failures in the thread join test on Windows

[mstorsjo]

Summary

Make sure that the detached thread has started up before exiting
the process.

This is exactly the same fix as D105592, with the same pattern
being present in a different test case.

[mstorsjo]

Add a missing <atomic> to appease the modules build.

[Mordante]

Thanks for fixing this issue!

libcxx/test/std/thread/thread.threads/thread.thread.class/thread.thread.member/join.pass.cpp

 #include <new>
 #include <cstdlib>
 #include <cassert>
 #include <system_error>
 
 #include "make_test_thread.h"
 #include "test_macros.h"
 
+std::atomic_bool done(false);
+
 class G
 {
     int alive_;
 public:
     static int n_alive;
     static bool op_run;
 
     G() : alive_(1) {++n_alive;}
     G(const G& g) : alive_(g.alive_) {++n_alive;}
     ~G() {alive_ = 0; --n_alive;}
 
     void operator()()
     {
         assert(alive_ == 1);
         assert(n_alive >= 1);
         op_run = true;
     }
 };
 
 int G::n_alive = 0;
 bool G::op_run = false;
 
-void foo() {}
+void foo() { done = true; }
 
 int main(int, char**)
 {
     {
         G g;
         std::thread t0 = support::make_test_thread(g);
         assert(t0.joinable());
         t0.join();
#ifndef TEST_HAS_NO_EXCEPTIONS
     {
         std::thread t0 = support::make_test_thread(foo);
         t0.detach();
         try {
             t0.join();
             assert(false);
         } catch (std::system_error const&) {
         }
+        // Wait to make sure that the detached thread has started up.
+        // Without this, we could exit main and start destructing global
+        // resources that are needed when the thread starts up, while the
+        // detached thread would start up only later.
+        while (!done) {}
     }
 #endif
 
   return 0;
 }