Modify generate_code of some Mcc class.

1. MccRobot: add some function for global declarations to report and some
members to trace the generation progress, such as m_current_func_decl.
2. MccFunctionDeclaration: add some function for local variable
declarations to report and some members to save states, such as
m_local_var_decls.
3. MccVariableDeclaration: change the implement of generate_code.
4. MccReturnStatement: change the implement of generate_code.