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/lints.rs

@@ -0,0 +1,14 @@
+use rustc_macros::HashStable_Generic;
+use rustc_span::Span;
+
+#[derive(Clone, Debug, HashStable_Generic)]
+pub struct AttributeLint<Id> {
+    pub id: Id,
+    pub span: Span,
+    pub kind: AttributeLintKind,
+}
+
+#[derive(Clone, Debug, HashStable_Generic)]
+pub enum AttributeLintKind {
+    UnusedDuplicate { this: Span, other: Span, warning: bool },
+}