cmd/gopherbot: post "CL nnnn mentions this issue." on recently closed issues
cmd/gopherbot: post "CL nnnn mentions this issue." on recently closed issues

Some CLs may refer to recently closed issues (e.g., fixups or
followups). It'd be better if @gopherbot would post the usual
"CL nnnn mentions this issue." comment in those situations.

Currently, the bot skips all closed issues when considering posting
that message. This was done to avoid posting in really old,
frozen issues (e.g., #1, #2, etc.) when they're accidentally
referenced.

However, a better proxy is to check whether the issue has the
FrozenDueToAge label, which the bot automatically applies to
old issues that are closed (see gopherbot.freezeOldIssues).

Fixes golang/go#21106.

Change-Id: If7489e8c001180944c47a0205909aa4043693436
Reviewed-on: https://go-review.googlesource.com/50470
Reviewed-by: Matt Layher <76528ff2e917253aa2211f68e4ca60b5b846dcf5@gmail.com>
Reviewed-by: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>
Run-TryBot: Matt Layher <76528ff2e917253aa2211f68e4ca60b5b846dcf5@gmail.com>

golang/build,golang/build,golang/build,golang/build