Did a lot more work on abtracting and organizing the platforms.

On Mac OS X we now have 3 platforms:
PlatformDarwin - must be subclassed to fill in the missing pure virtual funcs
                 but this implements all the common functionality between
                 remote-macosx and remote-ios. It also allows for another
                 platform to be used (remote-gdb-server for now) when doing
                 remote connections. Keeping this pluggable will allow for
                 flexibility.
PlatformMacOSX - Now implements both local and remote macosx desktop platforms.
PlatformRemoteiOS - Remote only iOS that knows how to locate SDK files in the
                    cached SDK locations on the host.

A new agnostic platform has been created:
PlatformRemoteGDBServer - this implements the platform using the GDB remote 
                          protocol and uses the built in lldb_private::Host
                          static functions to implement many queries.

llvm-svn: 128193