Support #[cfg]s on generic parameters

[jonas-schievink]

This (valid) Rust code currently shows type S<{unknown}> for s:

struct S<#[cfg(never)] T>(u8);

pub fn f() {
    let s: S;
}

This is because we ignore #[cfg] attributes on generic parameters.

To fix this, generic_params_query needs to be changed to filter out parameters whose #[cfg] expression evaluates to false, and it looks like the item tree needs to be changed to record these attributes.