[libc++] Fix the return value of max_size()

I assume nobody ever uses std::string_view::max_size() outside of
testing. However, we should still return a value that is based on
something with a reasonable rationale. Previously, we would forget
to take into account the size of the character type stored in the
string, and this patch takes that into account.

Thanks to @mclow.lists for pointing out this issue.

Differential Revision: https://reviews.llvm.org/D114395