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_interface/src/queries.rs

@@ -266,7 +266,7 @@ impl<'tcx> Queries<'tcx> {
         };
 
         let attrs = &*tcx.get_attrs(def_id);
-        let attrs = attrs.iter().filter(|attr| tcx.sess.check_name(attr, sym::rustc_error));
+        let attrs = attrs.iter().filter(|attr| attr.has_name(sym::rustc_error));
         for attr in attrs {
             match attr.meta_item_list() {
                 // Check if there is a `#[rustc_error(delay_span_bug_from_inside_query)]`.