Also change type from map[*types.Type]bool to map[*types.Type]struct{}.
This is basically a clean-up.

Change-Id: I167583eff0fa1070a7522647219476033b52b840
Reviewed-on: https://go-review.googlesource.com/41859Reviewed-by: Josh Bleecher Snyder <josharian@gmail.com>
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>