void create_directory_symlink(const path& to, const path& new_symlink);
void create_directory_symlink(const path& to, const path& new_symlink,
error_code& ec) noexcept;
Postconditions: new_symlink resolves to a symbolic link file that contains an unspecified representation of to.
Throws: As specified in [fs.err.report].
[ Note: Some operating systems require symlink creation to identify that the link is to a directory. Portable code should use create_directory_symlink() to create directory symlinks rather than create_symlink() — end note ]