use stable sort to sort multipart diagnostics

Author: folkertdev

compiler/rustc_errors/src/diagnostic.rs

@@ -920,7 +920,7 @@ impl<'a, G: EmissionGuarantee> Diag<'a, G> {
         applicability: Applicability,
         style: SuggestionStyle,
     ) -> &mut Self {
-        suggestion.sort_unstable();
+        suggestion.sort_by_key(|(span, _)| *span);
         suggestion.dedup_by(|(s1, m1), (s2, m2)| s1.source_equal(*s2) && m1 == m2);
 
         let parts = suggestion