fix: missing_const_for_fn FP on unstable const traits

[profetia]

Closes #14020
Closes #14290
Closes #14091

Add checks for unstable const traits.

changelog: [missing_const_for_fn] fix FP on unstable const traits

[rustbot]

r? @Alexendoo

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[profetia]

r? clippy

[samueltardieu]

You should add a separate "Closes #14290" line to the description so that the issue and the PR are linked.

[profetia]

r? clippy

[xFrednet]

I sadly don't have the time to review this rn. Sorry for bouncing it again 🙈

r? clippy

[y21]

The change LGTM, thanks!

There's still some issues with how qualify_min_const_fn deals with const traits (for example, the issue14290::f test case still gets a false positive warning when adding the feature gate and removing the msrv attribute; it needs a T: const Add bound in order to be a const fn which the code doesn't seem to account for).

However, I'm pretty sure that that is a pre-existing issue and this PR shouldn't be at fault for that (since this just adapts to the rustc change that const stability is not a property of trait methods anymore and is instead inherited from the parent trait afaict, and as such basically keeps the behavior that was there before the rustc change).