Auto merge of #123819 - joboet:fmt_usize_marker, r=Mark-Simulacrum

Get rid of `USIZE_MARKER` in formatting infrastructure

An alternative to #123780.

The `USIZE_MARKER` function used to differentiate between placeholder and count arguments is never called anyway, so we can just replace the function-pointer-comparison hack with an `enum` and an `unreachable_unchecked`, hopefully without causing a regression.

CC `@RalfJung`