Fix an issue where the @lldb.command marker would not work with the new...

Fix an issue where the @lldb.command marker would not work with the new 5-argument version of the Python command function

This:
a) teaches PythonCallable to look inside a callable object
b) teaches PythonCallable to discover whether a callable method is bound
c) teaches lldb.command to dispatch to either the older 4 argument version or the newer 5 argument version

llvm-svn: 273640