Rollup merge of #129659 - RalfJung:const-fn-lang-feat, r=fee1-dead

const fn stability checking: also check declared language features

Fixes rust-lang/rust#129656

`@oli-obk` I assume it is just an oversight that this didn't use `features().declared()`? Or is there a deep reason that this must only check `declared_lib_features`?

Author: matthiaskrgr