I've tested with clang 3.6.  The builder is running 3.5, and fails.

Fixes #12814.

Change-Id: I087fb75c3a24bed7f7fa5e9d7a1444590a316d63
Reviewed-on: https://go-review.googlesource.com/15259Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>