c++: Partially implement P1042R1: __VA_OPT__ wording clarifications [PR92319]

c++: Partially implement P1042R1: __VA_OPT__ wording clarifications [PR92319]

I've noticed we claim in cxx-status.html that we implement P1042R1,
but it seems we don't implement any of the changes from there.
The following patch implements just the change that __VA_OPT__ determines
whether to expand to nothing or the enclosed tokens no longer based on
whether there were any tokens passed to __VA_ARGS__, but whether __VA_ARGS__
expands to any tokens (from testing apparently it has to be non-CPP_PADDING
tokens).

I'm afraid I'm completely lost about the padding preservation/removal
changes that are also in the paper, so haven't touched that part.

2020-02-14  Jakub Jelinek  <jakub@redhat.com>

	Partially implement P1042R1: __VA_OPT__ wording clarifications
	PR preprocessor/92319
	* macro.c (expand_arg): Move declarations before vaopt_state
	definition.
	(class vaopt_state): Move enum update_type definition earlier.  Remove
	m_allowed member, add m_arg and m_update members.
	(vaopt_state::vaopt_state): Change last argument from bool any_args
	to macro_arg *arg, initialize m_arg and m_update instead of m_allowed.
	(vaopt_state::update): When bumping m_state from 1 to 2 and m_update
	is ERROR, determine if __VA_ARGS__ expansion has any non-CPP_PADDING
	tokens and set m_update to INCLUDE if it has any, DROP otherwise.
	Return m_update instead of m_allowed ? INCLUDE : DROP in m_state >= 2.
	(replace_args, create_iso_definition): Adjust last argument to
	vaopt_state ctor.

	* c-c++-common/cpp/va-opt-4.c: New test.