c++: modules don't require preprocessor output

c++: modules don't require preprocessor output

init_modules has rejected -M -fmodules-ts on the premise that module
dependency analysis requires macro expansion, but this is no longer
accurate; P1857 prohibited module directives produced by macro expansion.
They can still be dependent on #if directives, but those are still handled
with -fdirectives-only.

What wasn't working was -M or -dM, because cpp_scan_nooutput never called
module_token_pre to implement the import.  The simplest fix is to use the
-fdirectives-only scan when modules are enabled and teach directives_only_cb
about flag_no_output.

gcc/cp/ChangeLog:

	* module.cc (init_modules): Don't warn about -M.

gcc/c-family/ChangeLog:

	* c-ppoutput.cc (preprocess_file): For modules,
	use directives-only scan even with flag_no_output.
	(directives_only_cb): Respect flag_no_output.

gcc/ChangeLog:

	* doc/invoke.texi (C++ Module Preprocessing): Allow -M,
	refer to -fdeps.

gcc/testsuite/ChangeLog:

	* g++.dg/modules/macro-8_a.H: New test.
	* g++.dg/modules/macro-8_b.C: New test.
	* g++.dg/modules/macro-8_c.C: New test.
	* g++.dg/modules/macro-8_d.C: New test.