Remove Session.used_attrs and move logic to CheckAttrVisitor

Instead of updating global state to mark attributes as used,
we now explicitly emit a warning when an attribute is used in
an unsupported position. As a side effect, we are to emit more
detailed warning messages (instead of just a generic "unused" message).

`Session.check_name` is removed, since its only purpose was to mark
the attribute as used. All of the callers are modified to use
`Attribute.has_name`

Additionally, `AttributeType::AssumedUsed` is removed - an 'assumed
used' attribute is implemented by simply not performing any checks
in `CheckAttrVisitor` for a particular attribute.

We no longer emit unused attribute warnings for the `#[rustc_dummy]`
attribute - it's an internal attribute used for tests, so it doesn't
mark sense to treat it as 'unused'.

With this commit, a large source of global untracked state is removed.

compiler/rustc_passes/src/diagnostic_items.rs

@@ -15,7 +15,6 @@ use rustc_hir as hir;
 use rustc_hir::itemlikevisit::ItemLikeVisitor;
 use rustc_middle::ty::query::Providers;
 use rustc_middle::ty::TyCtxt;
-use rustc_session::Session;
 use rustc_span::def_id::{CrateNum, DefId, LocalDefId, LOCAL_CRATE};
 use rustc_span::symbol::{sym, Symbol};
 
@@ -51,7 +50,7 @@ impl<'tcx> DiagnosticItemCollector<'tcx> {
     fn observe_item(&mut self, def_id: LocalDefId) {
         let hir_id = self.tcx.hir().local_def_id_to_hir_id(def_id);
         let attrs = self.tcx.hir().attrs(hir_id);
-        if let Some(name) = extract(&self.tcx.sess, attrs) {
+        if let Some(name) = extract(attrs) {
             // insert into our table
             collect_item(self.tcx, &mut self.items, name, def_id.to_def_id());
         }
@@ -91,10 +90,10 @@ fn collect_item(
     }
 }
 
-/// Extract the first `rustc_diagnostic_item = "$name"` out of a list of attributes.
-fn extract(sess: &Session, attrs: &[ast::Attribute]) -> Option<Symbol> {
+/// Extract the first `rustc_diagnostic_item = "$name"` out of a list of attributes.p
+fn extract(attrs: &[ast::Attribute]) -> Option<Symbol> {
     attrs.iter().find_map(|attr| {
-        if sess.check_name(attr, sym::rustc_diagnostic_item) { attr.value_str() } else { None }
+        if attr.has_name(sym::rustc_diagnostic_item) { attr.value_str() } else { None }
     })
 }