go/types, types2: implement underIs, coreType, coreString via typeset iterator

go/types, types2: implement underIs, coreType, coreString via typeset iterator

Remove remaining underIs methods and call underIs function instead.

Change-Id: Ic98430d3a56b85f6f4b35c4508c4c67dafbfa3f2
Reviewed-on: https://go-review.googlesource.com/c/go/+/614240
Reviewed-by: Tim King <taking@google.com>
Reviewed-by: Robert Griesemer <gri@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Auto-Submit: Robert Griesemer <gri@google.com>