[libc++] Simplify std::ranges::subrange

Instead of using a base class to store the members and the optional
size, use [[no_unique_address]] to achieve the same thing without
needing a base class.

Also, as a fly-by:
- Change subrange from struct to class (per the standard)
- Improve the diagnostic for when one doesn't provide a size to the ctor of a sized subrange
- Replace this->member by just member since it's not in a dependent base anymore

This change would be an ABI break due to [[no_unique_address]], but we
haven't shipped ranges anywhere yet, so this shouldn't affect anyone.

Differential Revision: https://reviews.llvm.org/D110370