Add clippy into the known cfg list

Author: GuillaumeGomez

compiler/rustc_session/src/config.rs

@@ -1441,6 +1441,7 @@ impl CheckCfg {
         // These three are never set by rustc, but we set them anyway: they
         // should not trigger a lint because `cargo doc`, `cargo test`, and
         // `cargo miri run` (respectively) can set them.
+        ins!(sym::clippy, no_values);
         ins!(sym::doc, no_values);
         ins!(sym::doctest, no_values);
         ins!(sym::miri, no_values);