Fall back to being const-unstable when undeclared

[jhpratt]

This requires a method to have #[rustc_const_stable] to be const-stable. If no attribute is present, it is no longer assumed to be stable.

This is a follow-up on #90998 suggested by @RalfJung. I have not included any test for this, as it should never be encountered once #90998 is merged. If there's a situation where a test could work, that situation should be validated rather than adding a test for this.

r? @oli-obk

@rustbot label +A-attributes +A-const-fn +C-enhancement +S-waiting-on-review

[jhpratt]

@rust-log-analyzer is oddly quiet, but it looks like I need to consider const trait implementations.

@rustbot label -S-waiting-on-review +S-waiting-on-author

[jhpratt]

I think this is going to be a bit more involved than I initially thought. My current intent is to have a PR that will do this and perform a proper stability check for impl const Trait. I believe the latter will make this simpler, and is more in line with what we want anyways.