go/types, types2: remove remaining mentions of core type in error messages
The implementatiom still calls coreType in places and refers to
"core types" in comments, but user-visible error messages don't
know about core types anymore.
This brings the user-visible part of the implementation in sync with
the spec which doesn't have the notion of core types anymore.
For #70128.
Change-Id: I14bc6767a83e8f54b10ebe99a7df0b98cd9fca87
Reviewed-on: https://go-review.googlesource.com/c/go/+/654395
Auto-Submit: Robert Griesemer <gri@google.com> Reviewed-by: Robert Findley <rfindley@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com> Reviewed-by: Robert Griesemer <gri@google.com>