[libFuzzer][MSVC] Enable building libFuzzer with MSVC

Summary:
Enable building libFuzzer with MSVC.

* Don't try to include <endian.h> in FuzzerSHA1.cpp. MSVC
  doesn't have this header, and WINDOWS is always little
  endian (even on ARM)

Subscribers: srhines, mgorny, javed.absar, kristof.beyls

Differential Revision: https://reviews.llvm.org/D56510

llvm-svn: 351855