cmd/vendor: update to golang.org/x/tools@3ef68632

Mainly to pull in the bug fix in the structtag pass, where filenames
could sometimes be wrong. The bug wasn't present in 1.11, so it was a
regression and needs fixing before 1.12 is out.

Fixes #29130.

Change-Id: Ie9d9bff84873f34d748ebd8f056b6bc2ac822a55
Reviewed-on: https://go-review.googlesource.com/c/156378


Run-TryBot: Daniel Martí <mvdan@mvdan.cc>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Alan Donovan <adonovan@google.com>