[libc++] [API BREAK] Change `fs::path::iterator::iterator_category` to `input_iterator_tag`.

This essentially reverts e02ed1c2 and puts in a new fix, which makes `path::iterator`
a true C++20 `bidirectional_iterator`, but downgrades it to an `input_iterator` in C++17.

Fixes #37852.

Differential Revision: https://reviews.llvm.org/D116489