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
It was missing the cast to `bool` in `bool(__t.empty())`.
It was wrongly using `std::forward` in some places.
Differential Revision: https://reviews.llvm.org/D115312