Solve Visual C++ warning C4805 on getAsInteger<bool>.

Fix http://llvm.org/PR21158 by adding a cast to unsigned long long,
so the comparison would be between two unsigned long longs instead
of bool and unsigned long long.

      if (getAsUnsignedInteger(*this, Radix, ULLVal) ||
          static_cast<unsigned long long>(static_cast<T>(ULLVal)) != ULLVal)