go/types, types2: move Checker.langCompat from version.go to expr.go (cleanup)
This makes version.go holding core version checking code only.
No functional changes.
Change-Id: Ia88a48166cad2698765697dd7a8625b56ecc2226
Reviewed-on: https://go-review.googlesource.com/c/go/+/567536 Reviewed-by: Robert Griesemer <gri@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com> Reviewed-by: Robert Findley <rfindley@google.com>
Auto-Submit: Robert Griesemer <gri@google.com>