coverage: Simplify grouping of mappings by file

This removes an ad-hoc implementation of `group_by`.

compiler/rustc_codegen_llvm/src/lib.rs

@@ -10,6 +10,7 @@
 #![feature(iter_intersperse)]
 #![feature(let_chains)]
 #![feature(never_type)]
+#![feature(slice_group_by)]
 #![feature(impl_trait_in_assoc_type)]
 #![recursion_limit = "256"]
 #![allow(rustc::potential_query_instability)]