[ProcessWindows] Implement breakpoint stop / resume on Windows.

This patch implements basic support for stopping at breakpoints
and resuming later.  While a breakpoint is stopped at, LLDB will
cease to process events in the debug loop, effectively suspending
the process, and then resume later when ProcessWindows::DoResume
is called.

As a side effect, this also correctly handles the loader breakpoint
(i.e. the initial stop) so that LLDB goes through the correct state
sequence during the initial process launch.