go/loader: issue informative error (not panic) if cgo used in ad hoc package

See https://github.com/golang/go/issues/11627.

Change-Id: I458bc4ea54d0db34f3ba96060d284eda4bad7111
Reviewed-on: https://go-review.googlesource.com/12190
Reviewed-by: David Crawshaw <crawshaw@golang.org>
1 file changed