Function sync

test/stdexec/types/test_task.cpp:253–256

#  if !STDEXEC_GCC()                                                                               \
    || (defined(__OPTIMIZE__) && !defined(__SANITIZE_ADDRESS__) && !defined(__SANITIZE_THREAD__))
  auto sync() -> ex::task<int>
  {
    co_return 42;
  }

  auto nested() -> ex::task<int>
  {