tsan: add test for __cxa_atexit

[dvyukov]

Summary

Add a test for a common C++ bug when a global object is destroyed
while background threads still use it.

Depends on D114604.

compiler-rt/test/tsan/atexit5.cpp

+// RUN: %clangxx_tsan -O1 %s -o %t && %deflake %run %t | FileCheck %s
+#include "test.h"
+#include <memory>
+
+std::unique_ptr<long> global(new long(42));
+
+void *thread(void *x) {
+  *global = 43;
+  barrier_wait(&barrier);
+  return nullptr;
+}
+
+int main() {
+  barrier_init(&barrier, 2);
+  pthread_t th;
+  pthread_create(&th, nullptr, thread, nullptr);
+  pthread_detach(th);
+  barrier_wait(&barrier);
+  return 0;
+}
+
+// CHECK: WARNING: ThreadSanitizer: data race
+// CHECK:   Write of size 8
+// The exact spelling and number of std frames is hard to guess.
+// CHECK:     unique_ptr
+// CHECK:     #{{1|2}} cxa_at_exit_wrapper