mbe: Fix error message for using a macro with no attr rules as an attribute

Avoid saying "a declarative macro cannot be used as an attribute macro";
instead, say that the macro has no `attr` rules.

Author: joshtriplett

compiler/rustc_resolve/messages.ftl

@@ -246,7 +246,7 @@ resolve_lowercase_self =
     .suggestion = try using `Self`
 
 resolve_macro_cannot_use_as_attr =
-    `{$ident}` exists, but a declarative macro cannot be used as an attribute macro
+    `{$ident}` exists, but has no `attr` rules
 
 resolve_macro_cannot_use_as_derive =
      `{$ident}` exists, but a declarative macro cannot be used as a derive macro

tests/ui/macros/macro-rules-as-derive-or-attr-issue-132928.stderr

@@ -11,7 +11,7 @@ error: cannot find attribute `sample` in this scope
   --> $DIR/macro-rules-as-derive-or-attr-issue-132928.rs:5:3
    |
 LL | macro_rules! sample { () => {} }
-   |              ------ `sample` exists, but a declarative macro cannot be used as an attribute macro
+   |              ------ `sample` exists, but has no `attr` rules
 LL |
 LL | #[sample]
    |   ^^^^^^