go/types, types2: remove remaining references to coreType in literals.go

go/types, types2: remove remaining references to coreType in literals.go

For now, use commonUnder (formerly called sharedUnder) and update
error messages and comments. We can provide better error messages
in individual cases eventually.

For #70128.

Change-Id: I906ba9a0c768f6499c1683dc9be3ad27da8007a9
Reviewed-on: https://go-review.googlesource.com/c/go/+/653156
Reviewed-by: Robert Griesemer <gri@google.com>
Auto-Submit: Robert Griesemer <gri@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Robert Findley <rfindley@google.com>