Tweak thread names for CGU processing.

For non-incremental builds on Unix, currently all the thread names look
like `opt regex.f10ba03eb5ec7975-cgu.0`. But they are truncated by
`pthread_setname` to `opt regex.f10ba`, hiding the numeric suffix that
distinguishes them. This is really annoying when using a profiler like
Samply.

This commit changes these thread names to a form like `opt cgu.0`, which
is much better.

compiler/rustc_monomorphize/src/partitioning.rs

@@ -431,6 +431,9 @@ fn merge_codegen_units<'tcx>(
         codegen_units.sort_by_key(|cgu| cmp::Reverse(cgu.size_estimate()));
         let num_digits = codegen_units.len().ilog10() as usize + 1;
         for (index, cgu) in codegen_units.iter_mut().enumerate() {
+            // Note: `WorkItem::short_description` depends on this name ending
+            // with `-cgu.` followed by a numeric suffix. Please keep it in
+            // sync with this code.
             let suffix = format!("{index:0num_digits$}");
             let numbered_codegen_unit_name =
                 cgu_name_builder.build_cgu_name_no_mangle(LOCAL_CRATE, &["cgu"], Some(suffix));