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>
1 file changed