Fix nextPowTwo for 64-bit values under MSVC

Summary: A `long` is only 32-bits on MSVC, so this is simply wrong. Shift a `T` left instead.

Reviewed By: yfeldblum

Differential Revision: D3651139

fbshipit-source-id: 3bbfd18ed0c372287c4ec6cbcc543f6f1fcc4139

folly/Bits.h

@@ -185,7 +185,7 @@ typename std::enable_if<
   std::is_integral<T>::value && std::is_unsigned<T>::value,
   T>::type
 nextPowTwo(T v) {
-  return v ? (1ul << findLastSet(v - 1)) : 1;
+  return v ? (T(1) << findLastSet(v - 1)) : 1;
 }
 
 template <class T>