After Clang r313315, the static_assert errors look like:

  fatal error: static_assert failed due to requirement
  'param_is_forwardable' "Bound argument |i| of type |Arg| cannot be
  forwarded as [..]

instead of the previously expected:

  fatal error: static_assert failed "Bound argument |i| of type
  |Arg| cannot be forwarded as [..]

Since the test needs to work with both tip-of-tree and the currently
pinned version of Clang, add a regex to the expectations to make them
match both cases.

TBR=dcheng

Bug: 765692
Change-Id: I6271fcdb8f346ffd359dd98f4bfcb32f5fdc9480
Reviewed-on: https://chromium-review.googlesource.com/668572


Reviewed-by: Hans Wennborg <hans@chromium.org>
Commit-Queue: Hans Wennborg <hans@chromium.org>
Cr-Commit-Position: refs/heads/master@{#502315}