[lldb-vscode] Support running in server mode on Windows.

Windows can't use standard i/o system calls such as read and write
to work with sockets, it instead needs to use the specific send
and recv calls.  This complicates matters for the debug adapter,
since it needs to be able to work in both server mode where it
communicates over a socket, as well as non-server mode where it
communicates via stdin and stdout.  To abstract this out, I've
introduced a class IOStream which hides all these details and
exposes a read/write interface that does the right on each
platform.

Differential Revision: https://reviews.llvm.org/D59104

llvm-svn: 355637