go/types, types2: remove references to core type in append
Writing explicit code for this case turned out to be simpler
and easier to reason about then relying on a helper functions
(except for typeset).
While at it, make append error messages more consistent.
For #70128.
Change-Id: I3dc79774249929de5061b4301ab2506d4b3da0d0
Reviewed-on: https://go-review.googlesource.com/c/go/+/653095 Reviewed-by: Robert Findley <rfindley@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Auto-Submit: Robert Griesemer <gri@google.com> Reviewed-by: Robert Griesemer <gri@google.com>