substAny needs access to many internal details
of gc.Type. substArgTypes comes along for the ride.

Change-Id: I430a4edfd54a1266522f7a9818e5e7b5da72479c
Reviewed-on: https://go-review.googlesource.com/21250Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>