Allow registering tool lints with register_tool

Previously, there was no way to add a custom tool prefix, even if the tool
itself had registered a lint:

 ```
 #![feature(register_tool)]
 #![register_tool(xyz)]
 #![warn(xyz::my_lint)]
 ```

```
$ rustc unknown-lint.rs  --crate-type lib
error[E0710]: an unknown tool name found in scoped lint: `xyz::my_lint`
 --> unknown-lint.rs:3:9
  |
3 | #![warn(xyz::my_lint)]
  |         ^^^
```

This allows opting-in to lints from other tools using `register_tool`.

compiler/rustc_lint/src/context.rs

@@ -714,7 +714,7 @@ impl<'a> EarlyContext<'a> {
             sess,
             krate,
             lint_store,
-            builder: LintLevelsBuilder::new(sess, warn_about_weird_lints, lint_store),
+            builder: LintLevelsBuilder::new(sess, warn_about_weird_lints, lint_store, &krate.attrs),
             buffered,
         }
     }