Disallow `cfg` attribute (maybe others) on generic parameters

[Havvy]

As of 1.27, it is legal to put attributes on generic parameters. E.g. fn foo<#[allow(dead_code)] T>(_t: T){}, although none of the current attributes do anything. This includes attributes we may want to use in the future, such as cfg. To avoid doing the warning song and dance we did with #[must_use] on functions, we should disallow them before the feature actually stabilizes.

[kennytm]

cc #48848

Context of why it should be disallowed: https://botbot.me/mozilla/rust-lang/2018-06-01/?msg=100694737&page=1