Add support for stability attributes on foreign types

[carbotaniuman]

When implementing the FnPtr trait in #156176, I tried to implement Code as an extern type, however the stability infrastructure does not allow stability markers on extern types.

extern "C" {
    #[unstable(feature = "fn_static", issue = "148768")]
    type Opaque;
}

Gives the following error:

error: foreign type has missing stability attribute
   --> library/core/src/ops/function.rs:318:5
    |
318 |     pub type Code;
    |     ^^^^^^^^^^^^^

And if I try to put a stability marker on it I get this:

error: `#[unstable]` attribute cannot be used on foreign types
   --> library/core/src/ops/function.rs:318:5
    |
318 |     #[unstable(feature = "fn_static", issue = "148768")]
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    |
    = help: `#[unstable]` can be applied to associated consts, associated types, constants, crates, data types, enum variants, extern crates, foreign statics, functions, impl blocks, macro defs, modules, statics, struct fields, trait aliases, traits, type aliases, type parameters, and use statements

[JonathanBrouwer]

This should probably just work (though I haven't verified), we should just allow the target & add tests verifying it works

[kantnero]

@rustbot claim