libstdc++: Make non-propagating-cache fully constexpr [PR101263]

libstdc++-v3/ChangeLog:

	PR libstdc++/101263
	* include/std/ranges (__cached): New wrapper struct.
	(__non_propagating_cache): Use __cached for contained value.
	(__non_propagating_cache::_M_emplace_deref): Add constexpr. Use
	std::construct_at instead of placement new.
	* testsuite/std/ranges/adaptors/join.cc: Check constexpr works.