Always print lints from plugins, if they're available

Currently you can get a list of lints and lint groups by running `rustc
-Whelp`. This prints an additional line at the end:
```
Compiler plugins can provide additional lints and lint groups. To see a
listing of these, re-run `rustc -W help` with a crate filename.
```

Clippy is such a "compiler plugin", that provides additional lints.
Running `clippy-driver -Whelp` (`rustc` wrapper) still only prints the
rustc lints with the above message at the end. But when running
`clippy-driver -Whelp main.rs`, where `main.rs` is any rust file, it
also prints Clippy lints. I don't think this is a good approach from a
UX perspective: Why is a random file necessary to print a help message?

This commit changes this behavior: Whenever a compiler callback
registers lints, it is assumed that these lints come from a plugin and
are printed without having to specify a Rust source file.

compiler/rustc_driver/src/lib.rs

@@ -248,11 +248,18 @@ fn run_compiler(
                 interface::run_compiler(config, |compiler| {
                     let sopts = &compiler.session().opts;
                     if sopts.describe_lints {
-                        let lint_store = rustc_lint::new_lint_store(
+                        let mut lint_store = rustc_lint::new_lint_store(
                             sopts.debugging_opts.no_interleave_lints,
                             compiler.session().unstable_options(),
                         );
-                        describe_lints(compiler.session(), &lint_store, false);
+                        let registered_lints =
+                            if let Some(register_lints) = compiler.register_lints() {
+                                register_lints(compiler.session(), &mut lint_store);
+                                true
+                            } else {
+                                false
+                            };
+                        describe_lints(compiler.session(), &lint_store, registered_lints);
                         return;
                     }
                     let should_stop = RustcDefaultCalls::print_crate_info(