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/layout_test.rs

@@ -27,7 +27,7 @@ impl ItemLikeVisitor<'tcx> for LayoutTest<'tcx> {
             | ItemKind::Struct(..)
             | ItemKind::Union(..) => {
                 for attr in self.tcx.get_attrs(item.def_id.to_def_id()).iter() {
-                    if self.tcx.sess.check_name(attr, sym::rustc_layout) {
+                    if attr.has_name(sym::rustc_layout) {
                         self.dump_layout_of(item.def_id, item, attr);
                     }
                 }