c++: Add C++20 #__VA_OPT__ support

c++: Add C++20 #__VA_OPT__ support

The following patch implements C++20 # __VA_OPT__ (...) support.
Testcases cover what I came up with myself and what LLVM has for #__VA_OPT__
in its testsuite and the string literals are identical between the two
compilers on the va-opt-5.c testcase.

2021-08-17  Jakub Jelinek  <jakub@redhat.com>

libcpp/
	* macro.c (vaopt_state): Add m_stringify member.
	(vaopt_state::vaopt_state): Initialize it.
	(vaopt_state::update): Overwrite it.
	(vaopt_state::stringify): New method.
	(stringify_arg): Replace arg argument with first, count arguments
	and add va_opt argument.  Use first instead of arg->first and
	count instead of arg->count, for va_opt add paste_tokens handling.
	(paste_tokens): Fix up len calculation.  Don't spell rhs twice,
	instead use %.*s to supply lhs and rhs spelling lengths.  Don't call
	_cpp_backup_tokens here.
	(paste_all_tokens): Call it here instead.
	(replace_args): Adjust stringify_arg caller.  For vaopt_state::END
	if stringify is true handle __VA_OPT__ stringification.
	(create_iso_definition): Handle # __VA_OPT__ similarly to # macro_arg.
gcc/testsuite/
	* c-c++-common/cpp/va-opt-5.c: New test.
	* c-c++-common/cpp/va-opt-6.c: New test.