[libc++] [ranges] Fix a missing auto(x) cast in ranges::data.

Also remove some bogus `std::forward`s. My impression is that these
forwards were actually harmless, because `ranges::begin(FWD(t))` is
always identical to `ranges::begin(t)` (except when it's ill-formed,
and that can't happen in this case). However, they're also superfluous
and don't reflect the wording in the standard, so let's eliminate them.

Differential Revision: https://reviews.llvm.org/D117043