Create a process launcher abstraction.

This implements Host::LaunchProcess for windows, and in doing so
does some minor refactor to move towards a more modular process
launching design.

The original motivation for this is that launching processes on
windows needs some very windows specific code, which would live
most appropriately in source/Host/windows somewhere.  However,
there is already some common code that all platforms use when
launching a process before delegating to the platform specific
stuff, which lives in source/Host/common/Host.cpp which would
be nice to reuse without duplicating.

This commonality has been abstracted into MonitoringProcessLauncher,
a class which abstracts out the notion of launching a process using
an arbitrary algorithm, and then monitoring it for state changes.

The windows specific launching code lives in ProcessLauncherWindows,
and the posix specific launching code lives in ProcessLauncherPosix.
When launching a process MonitoringProcessLauncher is created, and
then an appropriate delegate launcher is created and given to the
MonitoringProcessLauncher.

Reviewed by: Greg Clayton
Differential Revision: http://reviews.llvm.org/D5781