Forbid attrs on macros, since we don't handle them properly yet.

src/test/compile-fail/attr-before-ext.rs

@@ -1,4 +1,4 @@
 fn main() {
     #[attr]
-    debug!("hi"); //~ ERROR expected item
+    debug!("hi"); //~ ERROR expected item after attrs
 }

src/test/compile-fail/ext-after-attrib.rs

@@ -1,4 +1,4 @@
-// error-pattern:expected item but found `fmt`
+// error-pattern:attrs on macros are not yet supported
 
 // Don't know how to deal with a syntax extension appearing after an
 // item attribute. Probably could use a better error message.