The change seems to have stuck, so bring us closer to C/++11 static asserts.

(If we later find we need to support worse toolchains, we can always use
__LINE__ or __COUNTER__ to avoid duplicate typedef names and just punt on
embedding the message into the type name.)

Change-Id: I0e5bb1106405066f07740728e19ebe13cae3e0ee
Reviewed-on: https://boringssl-review.googlesource.com/c/33145
Commit-Queue: Adam Langley <agl@google.com>
Reviewed-by: Adam Langley <agl@google.com>
CQ-Verified: CQ bot account: commit-bot@chromium.org <commit-bot@chromium.org>