make conversion explicit in `findLastSet` for longer integral types

Summary:
When the param type is larger than `unsigned int`, the wider type `size{}` was
implicitly converted to the return type `unsigned int` in `findLastSet`. This
diff makes the conversion explicit.

Reviewed By: yfeldblum

Differential Revision: D8971589

fbshipit-source-id: 8828504c462e9296b84a746bcb7f701bc4a7d20e

folly/lang/Bits.h

@@ -122,7 +122,7 @@ inline FOLLY_INTRINSIC_CONSTEXPR unsigned int findLastSet(T const v) {
   // allows GCC to remove its own xor that it adds to implement clz using bsr.
   // clang-format off
   using size = index_constant<constexpr_max(sizeof(T), sizeof(U0))>;
-  return v ? 1u + ((8u * size{} - 1u) ^ static_cast<unsigned int>(
+  return v ? 1u + static_cast<unsigned int>((8u * size{} - 1u) ^ (
       sizeof(T) <= sizeof(U0) ? __builtin_clz(bits_to_unsigned<U0>(v)) :
       sizeof(T) <= sizeof(U1) ? __builtin_clzl(bits_to_unsigned<U1>(v)) :
       sizeof(T) <= sizeof(U2) ? __builtin_clzll(bits_to_unsigned<U2>(v)) :