Add comment about why the regular unused-attributes infrastructure is not used for #[rustc_dirty]/#[rustc_clean].
Add comment about why the regular unused-attributes infrastructure
is not used for #[rustc_dirty]/#[rustc_clean].

GBGamer/rust,graydon/rust,aidancully/rust,graydon/rust,GBGamer/rust,graydon/rust,aidancully/rust,graydon/rust,graydon/rust,aidancully/rust,GBGamer/rust,aidancully/rust,aidancully/rust,GBGamer/rust,aidancully/rust,GBGamer/rust,GBGamer/rust,GBGamer/rust,GBGamer/rust,graydon/rust