Revert "go/types, types2: remove internal constant enableInterfaceInference"

Revert "go/types, types2: remove internal constant enableInterfaceInference"

This reverts CL 514715.

This will make it easier to make interface inference conditional
based on the current language version.

For #61903.

Change-Id: I07820c861d6ebfd04899e41eb4123f26af2da1ee
Reviewed-on: https://go-review.googlesource.com/c/go/+/520195
Reviewed-by: Robert Findley <rfindley@google.com>
Auto-Submit: Robert Griesemer <gri@google.com>
Reviewed-by: Robert Griesemer <gri@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Robert Griesemer <gri@google.com>