libstdc++: Add __maybe_const_t and __maybe_empty_t aliases

libstdc++: Add __maybe_const_t and __maybe_empty_t aliases

This introduces a couple of convenience alias templates to be used for
some repeated patterns using std::conditional_t.

	* include/std/ranges (__detail::__maybe_empty_t): Define new helper
	alias.
	(__detail::__maybe_const_t): Likewise.
	(__adaptor::_RangeAdaptor): Use __maybe_empty_t.
	(transform_view, take_view, take_while_view, elements_view): Use
	__maybe_const_t.
	(join_view, split_view): Use both.