[ProcessWindows] Simplify the DebugDelegate interface.

Due to a previous multi-threaded design involving message
passing, we used message classes to pass event information
to the delegate.  Since the multi-threaded design has gone
away, we simplify this by passing event arguments as direct
function parameters, which is more clear and easier to
understand.
[ProcessWindows] Improve support for launching processes.

This sends notifications for module load / unload to the process
plugin, and also manages the state more accurately during the
loading sequence.

Similar work by Virgile Bello was referenced during the
implementation of this patch.

Differential Revision: http://reviews.llvm.org/D6224