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_mir_dataflow/src/lib.rs

@@ -1,10 +1,10 @@
 // tidy-alphabetical-start
+#![cfg_attr(bootstrap, feature(let_chains))]
 #![feature(assert_matches)]
 #![feature(associated_type_defaults)]
 #![feature(box_patterns)]
 #![feature(exact_size_is_empty)]
 #![feature(file_buffered)]
-#![feature(let_chains)]
 #![feature(never_type)]
 #![feature(try_blocks)]
 // tidy-alphabetical-end