Summary:
Instead of ignoring UBSan checks, fix the negative shifts in
Hash(). Also add test to make sure the hash values are stable over
time. The values were computed before this change, so the test also
verifies the correctness of the change.
Closes https://github.com/facebook/rocksdb/pull/2546

Differential Revision: D5386369

Pulled By: yiwu-arbug

fbshipit-source-id: 6de4b44461a544d6222cc5d72d8cda2c0373d17e