constinit of instance_data<parallel_scheduler_backend> not conformant

[mathisloge]

the line inline constinit __instance_data<parallel_scheduler_backend, __parallel_scheduler_backend_impl>
__parallel_scheduler_backend_singleton{}; in here https://github.com/NVIDIA/stdexec/blob/954159ad82386b3564ea4125d9f4b7a68ccb912c/include/exec/__detail/__system_context_default_impl.hpp#L287C1-L288C46 wont compile with at least gcc 15, since the shared_ptr of instance_data has a non constexpr destructor

[ericniebler]

@lucteo could you have a look at this please?

[jehelset]

Seems to be fixed with: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=119652 (gcc-15.1.1, gcc-14.3.*). seems like making __instance_data members public is a workaroudn for gcc-14.2, didin't test on earlier gcc-15.