compiler: avoid aliases in receiver types

compiler: avoid aliases in receiver types

If a package declares a method on an alias type, the alias would be
used in the export data.  This would then trigger a compiler
assertion on import: we should not be adding methods to aliases.

Fix the problem by ensuring that receiver types do not use alias types.
This seems preferable to consistently avoiding aliases in export data,
as aliases can cross packages.  And it's painful to try to patch this
while writing the export data, as at that point all the types are known.

Test case is https://golang.org/cl/335172.

Fixes golang/go#47131

Reviewed-on: https://go-review.googlesource.com/c/gofrontend/+/335729