[lldb] Use WSAEventSelect for MainLoop polling on windows

This patch switches the MainLoop class to use the WSAEventSelect
mechanism to wait for multiple sockets to become readable. The
motivation for doing that is that this allows us to wait for other kinds
of events as well (as long as they can be converted to WSAEvents). This
will allow us to avoid (abstract away) pipe-based multiplexing
mechanisms in the generic code, since pipes cannot be combined with
sockets on windows.

Since the windows implementation will now look completely different than
the posix (file descriptor-based) implementations, I have split the
MainLoop class into two (MainLoopPosix and MainLoopWindows), with the
common code going into MainLoopBase.

Differential Revision: https://reviews.llvm.org/D131159