Fix bad regression from r263077 when building with MSVC.

That change did:

  -#if defined(__BIG_ENDIAN__)
  +#if __BYTE_ORDER__ == __ORDER_BIG_ENDIAN__

If __BYTE_ORDER__ and __ORDER_BIG_ENDIAN__ aren't defined, like
they are with MSVC, this condition is true (0 == 0).

Fixes PR26919.

llvm-svn: 263324