[libc++][ranges] Fixes as_rvalue's linkage.

[Mordante]

Summary

This was discovered while working on modules. They can't export
declarations with internal linkage.

[philnik]

Maybe we could add a clang-tidy check to make sure all variables in the std::ranges::__cpo namespace are marked inline?

[Mordante]

Thanks for the review!

In D149593#4309957, @philnik wrote:

Maybe we could add a clang-tidy check to make sure all variables in the std::ranges::__cpo namespace are marked inline?

That would be an extremely narrow check. I don't think that's too useful, especially since several similar issues were found be module tests. This tests validates the declarations in the headers and compares them in the modules. This is a useful test by itself and catches this error too.

libcxx/include/__ranges/as_rvalue_view.h

_LIBCPP_NODISCARD_EXT _LIBCPP_HIDE_FROM_ABI constexpr auto operator()(_Range&& __range) const
       noexcept(noexcept(/**/ views::all(std::forward<_Range>(__range))))
           -> decltype(/*--*/ views::all(std::forward<_Range>(__range))) {
     return /*-------------*/ views::all(std::forward<_Range>(__range));
   }
 };
 } // namespace __as_rvalue
 
 inline namespace __cpo {
-constexpr auto as_rvalue = __as_rvalue::__fn{};
+inline constexpr auto as_rvalue = __as_rvalue::__fn{};
 } // namespace __cpo
 } // namespace views
 } // namespace ranges
 
 _LIBCPP_END_NAMESPACE_STD
 
 #endif // _LIBCPP_STD_VER >= 23
 
 #endif // _LIBCPP___RANGES_AS_RVALUE_H