Use C++11 compatible version of static_assert; before C++17 the second message parameter to static_assert was not optional.
Use C++11 compatible version of static_assert; before
C++17 the second message parameter to static_assert was
not optional.

Some particularly old compiler (gcc-5) was having trouble
https://travis-ci.org/github/google/verible/jobs/669946250#L1428

PiperOrigin-RevId: 304449377

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