For example, `std::ranges::range<Holder<Incomplete>*>` should be
well-formed false, not a hard error at compile time.
Differential Revision: https://reviews.llvm.org/D116239
The big change here is that they now work as intended for rvalues,
e.g. `ranges::cbegin(std::string_view("hello"))`.
Also, add tests verifying their return types.
Differential Revision: https://reviews.llvm.org/D116199
Arthur O'Dwyer