Instead make substArgTypes responsible for cloning the function
definition Node and the function signature Type tree.

Passes toolstash -cmp.

Change-Id: I9ec84c90a7ae83d164d3f578e84a91cf1490d8ab
Reviewed-on: https://go-review.googlesource.com/20239
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>