preprocessor: Deferred macro support

preprocessor: Deferred macro support

For deferred macros we also need a new field on the macro itself, so
that the module machinery can determine the macro was imported.  Also
the documentation for the hashnode's deferred field was incomplete.

	libcpp/
	* include/cpplib.h (struct cpp_macro): Add imported_p field.
	(struct cpp_hashnode): Tweak deferred field documentation.
	* macro.c (_cpp_new_macro): Clear new field.
	(cpp_get_deferred_macro, get_deferred_or_lazy_macro): Assert
	more.