libcpp: Ignore CPP_PADDING tokens in _cpp_parse_expr [PR105732]

The first part of the following testcase (m1-m3 macros and its use)
regressed with my PR89971 fix, but as the m1,m4-m5 and its use part shows,
the problem isn't new, we can emit a CPP_PADDING token to avoid it from
being adjacent to whatever comes after the __VA_OPT__ (in this case there
is nothing afterwards, true).

In most cases these CPP_PADDING tokens don't matter, all other
callers of cpp_get_token_with_location either ignore CPP_PADDING tokens
completely (e.g. c_lex_with_flags) or they just remember them and
take them into account when printing stuff whether there should be
added whitespace or not (scan_translation_unit + token_streamer::stream).
So, I think we should just ignore CPP_PADDING tokens the same way in
_cpp_parse_expr.

2022-05-27  Jakub Jelinek  <jakub@redhat.com>

	PR preprocessor/105732
	* expr.cc (_cpp_parse_expr): Handle CPP_PADDING by just another
	token.

	* c-c++-common/cpp/va-opt-10.c: New test.