Add #![feature(let_else)]

Author: dswij

clippy_lints/src/lib.rs

@@ -8,6 +8,7 @@
 #![feature(rustc_private)]
 #![feature(stmt_expr_attributes)]
 #![feature(control_flow_enum)]
+#![feature(let_else)]
 #![recursion_limit = "512"]
 #![cfg_attr(feature = "deny-warnings", deny(warnings))]
 #![allow(clippy::missing_docs_in_private_items, clippy::must_use_candidate)]