All C++20 Ranges papers and LWG issues are done, with the exception of
https://wg21.link/P2210R2 ("Superior String Splitting"), and marked as
such.
All of these were already implemented prior to this patch except bumping
the feature test macro `__cpp_lib_ranges` as required by
https://wg21.link/P2325R3 ("Views should not be required to be default
constructible"). Note that, even though P2325R3 was voted into C++23, it
was voted with a recommendation for vendors to retroactively apply the
change to C++20 (see https://github.com/cplusplus/papers/issues/1007).
Differential Revision: https://reviews.llvm.org/D139900
Focus on the not-yet-implemented features: remove most details about the
already-implemented C++20 stuff, list out the major C++23 additions.
Differential Revision: https://reviews.llvm.org/D136657
