compiler: improve mixed named/unnamed parameter error message

Use the same error as the current gc compiler.

Change-Id: I03c6daf6f98b4535f59dcb16cfd6487cadba6c73
Reviewed-on: https://go-review.googlesource.com/c/gofrontend/+/273628
Trust: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Than McIntosh <thanm@google.com>
diff --git a/go/parse.cc b/go/parse.cc
index b062a47..00ac2f8 100644
--- a/go/parse.cc
+++ b/go/parse.cc
@@ -995,7 +995,7 @@
     }
   if (mix_error)
     {
-      go_error_at(location, "invalid named/anonymous mix");
+      go_error_at(location, "mixed named and unnamed function parameters");
       saw_error = true;
     }
   if (saw_error)