Implementing plugins that provide commands.

This checkin adds the capability for LLDB to load plugins from external dylibs that can provide new commands
It exports an SBCommand class from the public API layer, and a new SBCommandPluginInterface

There is a minimal load-only plugin manager built into the debugger, which can be accessed via Debugger::LoadPlugin.

Plugins are loaded from two locations at debugger startup (LLDB.framework/Resources/PlugIns and ~/Library/Application Support/LLDB/PlugIns) and more can be (re)loaded via the "plugin load" command

For an example of how to make a plugin, refer to the fooplugin.cpp file in examples/plugins/commands

Caveats:
	Currently, the new API objects and features are not exposed via Python.
	The new commands can only be "parsed" (i.e. not raw) and get their command line via a char** parameter (we do not expose our internal Args object)
	There is no unloading feature, which can potentially lead to leaks if you overwrite the commands by reloading the same or different plugins
	There is no API exposed for option parsing, which means you may need to use getopt or roll-your-own

llvm-svn: 164865