Rustc should warn if you have a stability attribute on a non-public item

[jyn514]

Originally posted by @ijackson in #83372 (comment)

[jhpratt]

For const fn this is sometimes necessary.

[camelid]

I'm not sure why this change doesn't work correctly, and I probably won't be able to look into it soon, so releasing assignment for now. I may try again in the future if someone else doesn't do it by then.

@rustbot claim