[libc++] Rename __fwd/hash.h to __fwd/functional.h and add reference_wrapper (#81445)

We forward declare `reference_wrapper` in multiple places already. This
moves the declaration to the canonical place and removes unnecessary
includes of `__functional/reference_wrapper.h`.

GitOrigin-RevId: 33de5a316caa3c9b07ee1ccc7fbc3434247ff787
19 files changed