c++: Add DECL_NTTP_OBJECT_P lang flag

c++: Add DECL_NTTP_OBJECT_P lang flag

VAR_DECLs for NTTPs need to be handled specially by module streaming,
in the same manner to type info decls.  This reworks their handling to
allow that work to drop in.  We use DECL_LANG_FLAG_5 to indicate such
decls (I didn't notice template_parm_object_p, which looks at the
mangled name -- anyway a bit flag on the node is better, IMHO).  We
break apart the creation routine, so there's now an entry point the
module machinery can use directly.

	gcc/cp/
	* cp-tree.h (DECL_NTTP_OBJECT_P): New.
	(template_parm_object_p): Delete.
	(build_template_parm_object): Declare.
	* cxx-pretty-print.cc (pp_cx_template_argument_list): Use DECL_NTTP_OBJECT_P.
	* error.cc (dump_simple_decl): Likewise.
	* mangle.cc (write_template_arg): Likewise.
	* pt.cc (template_parm_object_p): Delete.
	(create_template_parm_object): Separated out checking from ...
	(get_template_parm_object): ... this, new external entry point.