Avoid double-handling of attributes in collect_tokens.

By keeping track of attributes that have been previously processed.

This fixes the `macro-rules-derive-cfg.stdout` test, and is necessary
for #124141 which removes nonterminals.

Also shrink the `SmallVec` inline size used in `IntervalSet`. 2 gives
slightly better perf than 4 now that there's an `IntervalSet` in
`Parser`, which is cloned reasonably often.

compiler/rustc_index/src/interval.rs

@@ -18,7 +18,7 @@ mod tests;
 #[derive(Debug, Clone)]
 pub struct IntervalSet<I> {
     // Start, end
-    map: SmallVec<[(u32, u32); 4]>,
+    map: SmallVec<[(u32, u32); 2]>,
     domain: usize,
     _data: PhantomData<I>,
 }