Use multispan suggestions more often

* Use more accurate span for `async move` suggestion
* Use more accurate span for deref suggestion
* Use `multipart_suggestion` more often

compiler/rustc_errors/src/diagnostic_builder.rs

@@ -301,6 +301,20 @@ impl<'a> DiagnosticBuilder<'a> {
         self
     }
 
+    /// See [`Diagnostic::multipart_suggestions()`].
+    pub fn multipart_suggestions(
+        &mut self,
+        msg: &str,
+        suggestions: impl Iterator<Item = Vec<(Span, String)>>,
+        applicability: Applicability,
+    ) -> &mut Self {
+        if !self.0.allow_suggestions {
+            return self;
+        }
+        self.0.diagnostic.multipart_suggestions(msg, suggestions, applicability);
+        self
+    }
+
     /// See [`Diagnostic::span_suggestion_short()`].
     pub fn span_suggestion_short(
         &mut self,