[libc++] Eliminate the `__function_like` helper.

As prefigured in the comments on D115315.
This gives us one unified style for all niebloids,
and also simplifies the modulemap.

Differential Revision: https://reviews.llvm.org/D116570

NOKEYCHECK=True
GitOrigin-RevId: 63a991d0358970d76700d084f05eb95cd29234c0
14 files changed