Add SubdiagnosticMessageOp as a trait alias.

It avoids a lot of repetition.

compiler/rustc_macros/src/diagnostics/subdiagnostic.rs

@@ -89,10 +89,7 @@ impl SubdiagnosticDeriveBuilder {
             gen impl rustc_errors::AddToDiagnostic for @Self {
                 fn add_to_diagnostic_with<__F>(self, #diag: &mut rustc_errors::Diagnostic, #f: __F)
                 where
-                    __F: core::ops::Fn(
-                        &mut rustc_errors::Diagnostic,
-                        rustc_errors::SubdiagnosticMessage
-                    ) -> rustc_errors::SubdiagnosticMessage,
+                    __F: rustc_errors::SubdiagnosticMessageOp,
                 {
                     #implementation
                 }