Rollup merge of #120833 - nnethercote:more-internal-emit_diagnostics-cleanups, r=oli-obk

More internal emit diagnostics cleanups

Miscellaneous improvements.

r? ``@oli-obk``

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);