Rollup merge of #140202 - est31:let_chains_feature_compiler, r=lcnr

Make #![feature(let_chains)] bootstrap conditional in compiler/

Let chains have been stabilized recently in #132833, so we can remove the gating from our uses in the compiler (as the compiler uses edition 2024).

compiler/rustc_macros/src/lib.rs

@@ -1,7 +1,7 @@
 // tidy-alphabetical-start
 #![allow(rustc::default_hash_types)]
+#![cfg_attr(bootstrap, feature(let_chains))]
 #![feature(if_let_guard)]
-#![feature(let_chains)]
 #![feature(never_type)]
 #![feature(proc_macro_diagnostic)]
 #![feature(proc_macro_span)]