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

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

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

Kepp using coreType for make built-in for now because it must accept
different channel types with non-conflicting directions and identical
element types. Added extra test cases.

While at it, rename sharedUnder, sharedUnderOrChan to commonUnder
and commonUnderOrChan, respectively (per suggestion from rfindley).

For #70128.

Change-Id: I11f3d5ce858746574f4302271d8cb763c2cdcf98
Reviewed-on: https://go-review.googlesource.com/c/go/+/653139
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>