`pub` should not be allowed on nested items

[matklad]

The following compiles without errors:

fn main() {
    pub struct S;
    let _ = S;
}

It probably should be an error to specify pub on a nested item, because it has no effect.

[petrochenkov]

Hey, we permitted this only a few months ago, it's too soon to make it an error again.
See #31776 for details.

[matklad]

Hm, perhaps a lint should be added? Originally I thought of the following example:

fn a() {
    pub extern fn foo() {}
}