[BoundsSafety] Support `counted_by` on `void *` (1-byte element semantics)

Enable counted_by attribute on void * pointers to align with upstream behavior.

### Changes needed:

- Clang codegen: Handle void * with counted_by
- Consteval: Treat void * as 1-byte element size

Upstream treats `void *` with `counted_by` as an array of 1-byte elements. Our implementation for `-fbounds-safety` should match this behavior to prevent divergence.