c: Allow $@` in GNU23/GNU2Y raw string delimiters [PR110343]

c: Allow $@` in GNU23/GNU2Y raw string delimiters [PR110343]

Aaron mentioned in the PR that late in C23 N3124 was adopted and
$@` are now part of basic character set.  The paper has been implemented
in GCC from what I can see, but we should allow for GNU23/2Y $@` in
raw string delimiters as well, like they are allowed for C++26, because
the delimiters can contain anything from basic character set but space,
()\, tab, form-feed, newline and backspace.

2025-04-24  Jakub Jelinek  <jakub@redhat.com>

	PR c++/110343
	* lex.cc (lex_raw_string): For C allow $@` in raw string delimiters
	if CPP_OPTION (pfile, low_ucns) i.e. for C23 and later.

	* gcc.dg/raw-string-1.c: New test.