compiler: unalias receiver type in export data

Test case is https://golang.org/cl/292009.

Change-Id: I21cc1f4040bf283bc6d376ec026bdd216d915d58
Reviewed-on: https://go-review.googlesource.com/c/gofrontend/+/291991
Trust: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Than McIntosh <thanm@google.com>
Reviewed-by: Cherry Zhang <cherryyz@google.com>