GCC raises issues with `-Werror=non-template-friend`

从 subprojects/wg21_p2300_std_execution/include/execution.hpp 的第 969-986 行开始，并使用 GCC 和 -Werror=non-template-friend 重现 Meson 构建。检查报告中的 friend 声明，并验证将此警告视为错误时项目仍能成功构建。

By default, repositories compiled using the meson build enable -Werror=non-template-friend and this causes the following errors:

../subprojects/wg21_p2300_std_execution/include/execution.hpp:969:66: error: friend declaration ‘void std::execution::__connect::tag_invoke(_Tag, std::execution::__connect::__completion<_Tag(_Args ...)>&&, _Args&& ...)’ declares a non-template function [-Werror=non-template-friend]
  969 |         friend void tag_invoke(_Tag, __completion&&, _Args&&...) noexcept;
      |                                                                  ^~~~~~~~
../subprojects/wg21_p2300_std_execution/include/execution.hpp:969:66: note: (if this is not what you intended, make sure the function template has already been declared and add ‘<>’ after the function name here)
../subprojects/wg21_p2300_std_execution/include/execution.hpp:978:76: error: friend declaration ‘std::execution::__connect::__debug_env_t<_Env> std::execution::__connect::tag_invoke(std::execution::__env::get_env_t, std::execution::__connect::__debug_receiver<_Env, std::execution::completion_signatures<_Sigs ...> >)’ declares a non-template function [-Werror=non-template-friend]
  978 |         friend __debug_env_t<_Env> tag_invoke(get_env_t, __debug_receiver) noexcept;
      |                                                                            ^~~~~~~~
../subprojects/wg21_p2300_std_execution/include/execution.hpp:985:71: error: friend declaration ‘void std::execution::__connect::tag_invoke(std::execution::__receivers::set_stopped_t, std::execution::__connect::__any_debug_receiver<_Env>&&)’ declares a non-template function [-Werror=non-template-friend]
  985 |         friend void tag_invoke(set_stopped_t, __any_debug_receiver&&) noexcept;
      |                                                                       ^~~~~~~~
../subprojects/wg21_p2300_std_execution/include/execution.hpp:986:80: error: friend declaration ‘std::execution::__connect::__debug_env_t<_Env> std::execution::__connect::tag_invoke(std::execution::__env::get_env_t, std::execution::__connect::__any_debug_receiver<_Env>)’ declares a non-template function [-Werror=non-template-friend]
  986 |         friend __debug_env_t<_Env> tag_invoke(get_env_t, __any_debug_receiver) noexcept;