Check attributes on pattern fields.

Attributes on pattern struct fields were not being checked for validity.
This adds the fields as HIR nodes so that the `CheckAttrVisitor` can
visit those nodes to check their attributes.

compiler/rustc_ast_lowering/src/index.rs

@@ -193,6 +193,11 @@ impl<'a, 'hir> Visitor<'hir> for NodeCollector<'a, 'hir> {
 
     fn visit_pat(&mut self, pat: &'hir Pat<'hir>) {
         self.insert(pat.span, pat.hir_id, Node::Pat(pat));
+        if let PatKind::Struct(_, fields, _) = pat.kind {
+            for field in fields {
+                self.insert(field.span, field.hir_id, Node::PatField(field));
+            }
+        }
 
         self.with_parent(pat.hir_id, |this| {
             intravisit::walk_pat(this, pat);