go/types, types2: pull up Unalias call to cover all of cycleFinder.typ
Without a test because it's unclear the situation can actually occur,
but the code is correct because it now mimics the behavior without
explicit Alias nodes.
For #67547.
Change-Id: I21a31af28880ca6d599fe465563d9574c26ed1f1
Reviewed-on: https://go-review.googlesource.com/c/go/+/588117
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