Fix-up r278299 for windows

FD_SETSIZE on windows limits the number of file descriptors, rather than their individual
magnitude (the underlying implementation uses an array rather than a bitset). This meant that the
assert in the SelectHelper was incorrect, and failing all the time. Fix that.

I am not sure whether this should be #ifdef MSVC, or #ifdef WINDOWS, but my feeling is that a
more posix-conforming implementation on windows would choose the bitset implementation, so I'm
sticking with the former.

llvm-svn: 278500