MSRV attribute is not scoped

[camsteffen]

Example:

#![feature(custom_inner_attributes)]

mod my_mod {
    #![clippy::msrv = "1.0.0"]
}

fn hi() {
    // the `manual_strip` lint is suppressed by the attribute in `my_mod`
    let s = "hello, world!";
    if s.starts_with("hello, ") {
        assert_eq!(s["hello, ".len()..].to_uppercase(), "WORLD!");
    }
}

If we don't want to support this sort of usage, maybe we can just warn or error when the attribute is found in a non-crate.

[hellow554]

There's this testfile https://github.com/rust-lang/rust-clippy/blob/master/tests/ui/min_rust_version_attr.rs and it seems that scoping is supported in some way.

It seems, that the inner attribute starts from that line on in the code, because if I move your hi function above my_mod the lint is triggered correctly.

I'm not sure if that's a restriction of the custom_inner_attributes feature or clippys fault.