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.