noexcept hash to avoid caching of hash code in std::unordered_set
noexcept hash to avoid caching of hash code in std::unordered_set

Summary: libstdc++'s std::unordered_set caches the computed hash values
if the hash functor's operator() is not marked noexcept.  That penalizes
it somewhat in this benchmark by increasing the memory footprint and
adding an extra branch on the hit path.

google/hashtable-benchmarks,google/hashtable-benchmarks,google/hashtable-benchmarks