folly: ubsan: cast to unsigned to avoid negative-left-shift
Summary: The result of E1 << E2 is E1 left-shifted E2 bit positions; vacated bits are filled with zeros. If E1 has a signed type and non-negative value, and E1 × 2E2 is representable in the result type, then that is the resulting value; otherwise, the behavior is undefined. The code assumed the signed left shift worked like an unsigned left shift, so make that explicit. Reviewed By: meyering Differential Revision: D3280325 fbshipit-source-id: 178b95ee36b7a1126a79bb825c2ad2ffa6fa9464
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