libcpp: __VA_OPT__ tweak

libcpp: __VA_OPT__ tweak

> We want to remove the latter <placemarker> but not the former one, and
> the patch adds the vaopt_padding_tokens counter for it to control
> how many placemarkers are removed on vaopt_state::END.
> As can be seen in #c1 and #c2 of the PR, I've tried various approaches,
> but neither worked out for all the cases except the posted one.

I notice that the second placemarker you mention is avoid_paste, which seems
relevant.  This seems to also work, at least it doesn't seem to break any of
the va_opt tests.

2021-09-01  Jason Merrill  <jason@redhat.com>

	* macro.c (replace_args): When __VA_OPT__ is on the LHS of ##,
	remove trailing avoid_paste tokens.