Support folly::assume under MSVC

Summary: Actually __assume when we assume.

Reviewed By: meyering

Differential Revision: D3336033

fbshipit-source-id: 6c931e9f832dd1d861d03e3ad8e308f612522401

folly/Assume.h

@@ -38,6 +38,8 @@ FOLLY_ALWAYS_INLINE void assume(bool cond) {
   __builtin_assume(cond);
 #elif defined(__GNUC__)
   if (!cond) { __builtin_unreachable(); }
+#elif defined(_MSC_VER)
+  __assume(cond);
 #else
   // Do nothing.
 #endif