fix attribute validation on associated items in traits

compiler/rustc_ast_passes/src/ast_validation.rs

@@ -1519,6 +1519,7 @@ impl<'a> Visitor<'a> for AstValidator<'a> {
                     generics,
                     body.as_deref(),
                 );
+                walk_list!(self, visit_attribute, &item.attrs);
                 self.visit_fn(kind, item.span, item.id);
             }
             AssocItemKind::Type(_) => {

tests/ui/attributes/validation-on-associated-items-issue-121537.rs

@@ -0,0 +1,7 @@
+trait MyTrait {
+    #[doc = MyTrait]
+    //~^ ERROR attribute value must be a literal
+    fn myfun();
+}
+
+fn main() {}

tests/ui/attributes/validation-on-associated-items-issue-121537.stderr

@@ -0,0 +1,8 @@
+error: attribute value must be a literal
+  --> $DIR/validation-on-associated-items-issue-121537.rs:2:13
+   |
+LL |     #[doc = MyTrait]
+   |             ^^^^^^^
+
+error: aborting due to 1 previous error
+