go/types, types2: factor out single commonUnder function

go/types, types2: factor out single commonUnder function

Combine commonUnder and commonUnderOrChan:
- Provide an optional cond(ition) function argument to commonUnder
  to establish additional type set conditions.
- Instead of a *Checker and *string argument for error reporting,
  return an error cause that is only allocated in the presence of
  an error.
- Streamline some error messages.

Replace all calls to coreType with calls to commonUnder.

Change-Id: I81ac86d0d532cddc09164309acced61d90718b44
Reviewed-on: https://go-review.googlesource.com/c/go/+/654455
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>
Reviewed-by: Robert Findley <rfindley@google.com>