Rollup merge of #129818 - RalfJung:const-stability, r=compiler-errors

make the const-unstable-in-stable error more clear

The default should be to add `rustc_const_unstable`, not `rustc_allow_const_fn_unstable`.

Also I discovered our check for missing const stability attributes on stable functions -- but strangely that check only kicks in for "reachable" functions. `check_missing_stability` checks for reachability since all reachable functions must have a stability attribute, but I would say if a function has `#[stable]` it should also have const-stability attributes regardless of reachability.

Author: matthiaskrgr