Auto merge of rust-lang#141415 - est31:let_chains_libcore, r=tgross35

Remove #![feature(let_chains)] from libcore

PR rust-lang#132833 has stabilized the let_chains feature. This PR removes the last occurrence from the library.

Split out of rust-lang#140966 as it caused breakage. Now we have a patch to the linux tree that fixes the build, so we update the linux tree to it.

cc Rust-for-Linux/linux#1163
cc rust-lang#140722

Author: bors

core/src/lib.rs

@@ -158,7 +158,6 @@
 #![feature(intra_doc_pointers)]
 #![feature(intrinsics)]
 #![feature(lang_items)]
-#![feature(let_chains)]
 #![feature(link_llvm_intrinsics)]
 #![feature(macro_metavar_expr)]
 #![feature(marker_trait_attr)]