introduce new lint infra

lint on duplicates during attribute parsing
To do this we stuff them in the diagnostic context to be emitted after
hir is constructed

compiler/rustc_attr_data_structures/src/lib.rs

@@ -8,6 +8,8 @@ mod attributes;
 mod stability;
 mod version;
 
+pub mod lints;
+
 use std::num::NonZero;
 
 pub use attributes::*;