go/types, types2: add Environment to Config

go/types, types2: add Environment to Config

Port to types2 and adjust compiler accordingly.

Change-Id: I2e72b151ef834977dca64cb2e62cedcac4e46062
Reviewed-on: https://go-review.googlesource.com/c/go/+/348578
Trust: Robert Griesemer <gri@golang.org>
Run-TryBot: Robert Griesemer <gri@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>
Reviewed-by: Robert Findley <rfindley@google.com>
TryBot-Result: Go Bot <gobot@golang.org>