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/weak_lang_items.rs

@@ -1,5 +1,6 @@
 //! Validity checking for weak lang items
 
+use rustc_ast::Attribute;
 use rustc_data_structures::fx::FxHashSet;
 use rustc_errors::struct_span_err;
 use rustc_hir as hir;
@@ -96,7 +97,7 @@ impl<'a, 'tcx, 'v> Visitor<'v> for Context<'a, 'tcx> {
     }
 
     fn visit_foreign_item(&mut self, i: &hir::ForeignItem<'_>) {
-        let check_name = |attr, sym| self.tcx.sess.check_name(attr, sym);
+        let check_name = |attr: &Attribute, sym| attr.has_name(sym);
         let attrs = self.tcx.hir().attrs(i.hir_id());
         if let Some((lang_item, _)) = lang_items::extract(check_name, attrs) {
             self.register(lang_item, i.span);