Remove dcx arg from ReportErrorExt::add_args.

Because it also has a `DiagnosticBuilder` arg, which contains a `dcx`
reference.

Also rename some `builder` variables as `diag`, because that's the usual
name.

compiler/rustc_const_eval/src/const_eval/error.rs

@@ -151,7 +151,7 @@ where
             let mut err = tcx.dcx().create_err(err);
 
             let msg = error.diagnostic_message();
-            error.add_args(tcx.dcx(), &mut err);
+            error.add_args(&mut err);
 
             // Use *our* span to label the interp error
             err.span_label(our_span, msg);