coverage: Encode function mappings without re-sorting them

The main change here is that `VirtualFileMapping` now uses an internal hashmap
to de-duplicate incoming global file IDs. That removes the need for
`encode_mappings_for_function` to re-sort its mappings by filename in order to
de-duplicate them.

(We still de-duplicate runs of identical filenames to save work, but this is
not load-bearing for correctness, so a sort is not necessary.)

compiler/rustc_codegen_llvm/src/lib.rs

@@ -8,13 +8,13 @@
 #![cfg_attr(not(bootstrap), feature(rustdoc_internals))]
 #![cfg_attr(not(bootstrap), doc(rust_logo))]
 #![doc(html_root_url = "https://doc.rust-lang.org/nightly/nightly-rustc/")]
+#![feature(exact_size_is_empty)]
 #![feature(extern_types)]
 #![feature(hash_raw_entry)]
 #![feature(iter_intersperse)]
 #![feature(let_chains)]
 #![feature(min_specialization)]
 #![feature(never_type)]
-#![feature(slice_group_by)]
 #![feature(impl_trait_in_assoc_type)]
 #![recursion_limit = "256"]
 #![allow(rustc::potential_query_instability)]