go/types, types2: combine version check with version error reporting

go/types, types2: combine version check with version error reporting

This removes the duplicate (and possible error-prone) versions
(once for test and once for error message) and simplifies code.

Adjusted multiple go/types call sites to match types2.

Renamed posFor to atPos in types2, for closer match with go/types
and to keep automatic generation of instantiate.go working.

Change-Id: Iff428fc742f305a65bb7d077b7e31b66df3b706d
Reviewed-on: https://go-review.googlesource.com/c/go/+/491715
Reviewed-by: Robert Griesemer <gri@google.com>
Run-TryBot: Robert Griesemer <gri@google.com>
Reviewed-by: Robert Findley <rfindley@google.com>
Auto-Submit: Robert Griesemer <gri@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>