go/types, types2: remove Checker.pos from types2 code - not needed anymore
In go/types, move field down in environment struct, rename it to
exprPos, and document use.
Updates #69673.
Change-Id: I355af1237f8cd731ad9706e6a5fce34b314978cc
Reviewed-on: https://go-review.googlesource.com/c/go/+/616316
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com> Reviewed-by: Alan Donovan <adonovan@google.com>
Auto-Submit: Robert Griesemer <gri@google.com> Reviewed-by: Robert Griesemer <gri@google.com>