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_ast_lowering/src/lib.rs

@@ -2281,7 +2281,7 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
                     synthetic: param
                         .attrs
                         .iter()
-                        .filter(|attr| self.sess.check_name(attr, sym::rustc_synthetic))
+                        .filter(|attr| attr.has_name(sym::rustc_synthetic))
                         .map(|_| hir::SyntheticTyParamKind::FromAttr)
                         .next(),
                 };