Start the correct host abstraction of process and thread.

Added a new "Host/Debug.h" which contains the pure virtual class definitions for NativeProcessProtocol and NativeThreadProtocol. These classes are host layer classes that, over time, every host that supports debugging will implement once.

Then the new "lldb-gdbserver" binary in the tools directory will be able to make a remote debugging binary from these native classes, and we will also be able to have a new lldb_private::Process class that implements native debugging using these classes.

So as soon as linux and MacOSX switch over to using this layer, everyone will get native and remote debugging.

This check-in is primarily to get the needed code in so that poeple can start trying to implement the NativeProcessProtocol and NativeThreadProtocol and use it in the GDBRemoteCommunicationServer class to implement a GDB server for remote debugging.