go/types, types2: remove need for coreString in signature.go

go/types, types2: remove need for coreString in signature.go

Also, add additional test cases for NewSignatureType
to check expected panic behavior.

Change-Id: If26cd81a2af384bf2084dd09119483c0584715c8
Reviewed-on: https://go-review.googlesource.com/c/go/+/655695
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Robert Griesemer <gri@google.com>
Auto-Submit: Robert Griesemer <gri@google.com>
Reviewed-by: Robert Findley <rfindley@google.com>