fixup! rustc_typeck: improve diagnostics for _ const/static declarations

Author: lundibundi

src/test/ui/error-codes/E0121.stderr

@@ -11,7 +11,10 @@ error[E0121]: the type placeholder `_` is not allowed within types on item signa
   --> $DIR/E0121.rs:3:13
    |
 LL | static BAR: _ = "test";
-   |             ^ not allowed in type signatures
+   |             ^
+   |             |
+   |             not allowed in type signatures
+   |             help: replace `_` with the correct return type: `&'static str`
 
 error: aborting due to 2 previous errors