NewInterface2 was introduced with https://go-review.googlesource.com/114317
which fixed #25301. Changed the name to NewInterfaceType to better match
Go naming styles, per discussion with @josharian, @iant, et al.

Change-Id: Ifa4708a5efd4f708295b33c3d20fdc5812e1b4fc
Reviewed-on: https://go-review.googlesource.com/120875Reviewed-by: Josh Bleecher Snyder <josharian@gmail.com>
Reviewed-by: Ian Lance Taylor <iant@golang.org>