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_lint/src/lib.rs

@@ -151,8 +151,6 @@ macro_rules! late_lint_passes {
                 // FIXME: Look into regression when this is used as a module lint
                 // May Depend on constants elsewhere
                 UnusedBrokenConst: UnusedBrokenConst,
-                // Uses attr::is_used which is untracked, can't be an incremental module pass.
-                UnusedAttributes: UnusedAttributes::new(),
                 // Needs to run after UnusedAttributes as it marks all `feature` attributes as used.
                 UnstableFeatures: UnstableFeatures,
                 // Tracks state across modules