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_ty_utils/src/lib.rs

@@ -6,6 +6,7 @@
 
 // tidy-alphabetical-start
 #![allow(internal_features)]
+#![cfg_attr(bootstrap, feature(let_chains))]
 #![doc(html_root_url = "https://doc.rust-lang.org/nightly/nightly-rustc/")]
 #![doc(rust_logo)]
 #![feature(assert_matches)]
@@ -13,7 +14,6 @@
 #![feature(box_patterns)]
 #![feature(if_let_guard)]
 #![feature(iterator_try_collect)]
-#![feature(let_chains)]
 #![feature(never_type)]
 #![feature(rustdoc_internals)]
 // tidy-alphabetical-end