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_incremental/src/persist/dirty_clean.rs

@@ -159,7 +159,7 @@ pub struct DirtyCleanVisitor<'tcx> {
 impl DirtyCleanVisitor<'tcx> {
     /// Possibly "deserialize" the attribute into a clean/dirty assertion
     fn assertion_maybe(&mut self, item_id: LocalDefId, attr: &Attribute) -> Option<Assertion> {
-        if !self.tcx.sess.check_name(attr, sym::rustc_clean) {
+        if !attr.has_name(sym::rustc_clean) {
             // skip: not rustc_clean/dirty
             return None;
         }
@@ -427,7 +427,7 @@ pub struct FindAllAttrs<'tcx> {
 
 impl FindAllAttrs<'tcx> {
     fn is_active_attr(&mut self, attr: &Attribute) -> bool {
-        if self.tcx.sess.check_name(attr, sym::rustc_clean) && check_config(self.tcx, attr) {
+        if attr.has_name(sym::rustc_clean) && check_config(self.tcx, attr) {
             return true;
         }