go/types, types2: don't lose alias information during unification
While at it, rename asTypeParam to asBoundTypeParam for clarity.
For #67547.
Fixes #67628.
Change-Id: I2f447c4cd4d72f5315fe9323d82fcb9bf33657c6
Reviewed-on: https://go-review.googlesource.com/c/go/+/588797 Reviewed-by: Robert Griesemer <gri@google.com>
Auto-Submit: Robert Griesemer <gri@google.com>
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Commit-Queue: Robert Griesemer <gri@google.com> Reviewed-by: Robert Findley <rfindley@google.com>