Forbid running Clippy on stage 0

Author: Kobzol

src/bootstrap/src/core/config/config.rs

@@ -1375,6 +1375,10 @@ impl Config {
                 eprintln!("ERROR: cannot document anything on stage 0. Use at least stage 1.");
                 exit!(1);
             }
+            (0, Subcommand::Clippy { .. }) => {
+                eprintln!("ERROR: cannot run clippy on stage 0. Use at least stage 1.");
+                exit!(1);
+            }
             _ => {}
         }