Improve code of loading plugins that provide cmnds

Summary:
 - Modified code that enables writing new user-defined commands
   and use them through LLDB CLI. Modifications are:

  -- Define the 'syntax' for each user-defined command
    --- Added an argument in SBCommandInterpreter::AddCommand()
        and SBCommand::AddCommand() API
    --- Allow passing syntax for each user-defined command
    --- Earlier, only 'help' could be defined and passed for commands

  -- Passed 'number of arguments' entered on CLI for user-defined commands
    --- Added an argument (number of options) in SBCommandPluginInterface::DoExecute()
        API to know the number of arguments passed for commands

  -- In CommandPluginInterfaceImplementation class:
    --- Make the data member m_backend a shared_ptr
    --- Avoids memory leaks of dynamically allocated SBCommandPluginInterface instances
        created in lldb::PluginInitialize() API

Signed-off-by: Abhishek Aggarwal <abhishek.a.aggarwal@intel.com>

Reviewers: jingham, granata.enrico, clayborg

Subscribers: labath, lldb-commits

Differential Revision: https://reviews.llvm.org/D22863

llvm-svn: 277125