compiler: look through aliases when looking for methods

Add a Type::is_alias method to remove some existing loops and avoid
adding a new one.

Test case is https://golang.org/cl/89935.

Fixes golang/go#23489

Change-Id: Ieec9bf57392ce77f56c9f42c6c6868741d21e86f
Reviewed-on: https://go-review.googlesource.com/89975
Reviewed-by: Than McIntosh <thanm@google.com>
2 files changed