Preparation for eliminating Prog-related globals.

Passes toolstash-check -all.

Updates #15756

Change-Id: Ia199fcb282cc3a84903a6e92a3ce342c5faba79c
Reviewed-on: https://go-review.googlesource.com/38409
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>