[libcxx] adds `iter_difference_t` and `iter_value_t`

Implements parts of:
    * P0896R4 The One Ranges Proposal

Depends on D99855.

Reviewed By: ldionne, #libc

Differential Revision: https://reviews.llvm.org/D99863

GitOrigin-RevId: 9816d43cff5ad7abb94eab94dcd205285675972a
6 files changed