Allow the unstable attribute on foreign type

[kantnero]

While working the FnPtr trait in #156176 @carbotaniuman was trying to implement Code as an extern type but got an error as the current stability infrastructure does not allow unstable attribute on foreign type. #158200

This PR fixes that by allowing the unstable attribute on ForeignTy and adds test to verify.

@rustbot r? @JonathanBrouwer

[rustbot]

Some changes occurred in compiler/rustc_attr_parsing

cc @jdonszelmann, @JonathanBrouwer

[rustbot]

Thanks for the pull request, and welcome! The Rust Project is excited to review your changes, and you should hear from @JonathanBrouwer (or someone else) some time within the next two weeks.

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