Rollup merge of #100970 - Xiretza:derive-multipart-suggestion, r=davidtwco

Allow deriving multipart suggestions

This turned into a bit more of a rewrite than I was initially hoping for... Still, I think the `SessionSubdiagnostic` derive is a little cleaner overall now, and closer to the `SessionDiagnostic` derive to make future code sharing easier.

r? ``@davidtwco``

compiler/rustc_errors/src/diagnostic.rs

@@ -686,19 +686,12 @@ impl Diagnostic {
         suggestion: Vec<(Span, String)>,
         applicability: Applicability,
     ) -> &mut Self {
-        assert!(!suggestion.is_empty());
-        self.push_suggestion(CodeSuggestion {
-            substitutions: vec![Substitution {
-                parts: suggestion
-                    .into_iter()
-                    .map(|(span, snippet)| SubstitutionPart { snippet, span })
-                    .collect(),
-            }],
-            msg: self.subdiagnostic_message_to_diagnostic_message(msg),
-            style: SuggestionStyle::CompletelyHidden,
+        self.multipart_suggestion_with_style(
+            msg,
+            suggestion,
             applicability,
-        });
-        self
+            SuggestionStyle::CompletelyHidden,
+        )
     }
 
     /// Prints out a message with a suggested edit of the code.