Structured suggestion for "missing `feature` in unstable fn call"

When encountering a call corresponding to an item marked as unstable behind a feature flag, provide a structured suggestion pointing at where in the crate the `#![feature(..)]` needs to be written.

```
error: `foobar` is not yet stable as a const fn
  --> $DIR/const-stability-attribute-implies-no-feature.rs:12:5
   |
LL |     foobar();
   |     ^^^^^^^^
   |
help: add `#![feature(const_foobar)]` to the crate attributes to enable
   |
LL + #![feature(const_foobar)]
   |
```

Fix #81370.

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