c++: more modules and -M

In r15-4119-gc877a27f04f648 I told preprocess_file to use the
directives-only scan with modules, but it seems that I also need to set the
cpp_option so that communication between _cpp_handle_directive and
scan_translation_unit_directives_only works properly in
c-c++-common/cpp/embed-6.c.

gcc/c-family/ChangeLog:

	* c-ppoutput.cc (preprocess_file): Set directives_only flag.