Rollup merge of #49525 - varkor:sort_by_cached_key-conversion, r=scottmcm

Use sort_by_cached_key where appropriate

A follow-up to https://github.com/rust-lang/rust/pull/48639, converting various slice sorting calls to `sort_by_cached_key` when the key functions are more expensive.

src/libsyntax/lib.rs

@@ -21,6 +21,7 @@
 
 #![feature(unicode)]
 #![feature(rustc_diagnostic_macros)]
+#![feature(slice_sort_by_cached_key)]
 #![feature(non_exhaustive)]
 #![feature(const_atomic_usize_new)]
 #![feature(rustc_attrs)]