c++: Implement C++26 P3176R1 - The Oxford variadic comma

c++: Implement C++26 P3176R1 - The Oxford variadic comma

While we are already in stage3, I wonder if implementing this small paper
wouldn't be useful even for GCC 15, so that we have in the GCC world one
extra year of deprecation of variadic ellipsis without preceding comma.

The paper just deprecates something, I'd hope most of the C++ code in the
wild when it uses variadic functions at all uses the comma before the
ellipsis.

2024-11-30  Jakub Jelinek  <jakub@redhat.com>

gcc/c-family/
	* c.opt (Wdeprecated-variadic-comma-omission): New option.
	* c.opt.urls: Regenerate.
	* c-opts.cc (c_common_post_options): Default to
	-Wdeprecated-variadic-comma-omission for C++26 or -Wpedantic.
gcc/cp/
	* parser.cc: Implement C++26 P3176R1 - The Oxford variadic comma.
	(cp_parser_parameter_declaration_clause): Emit
	-Wdeprecated-variadic-comma-omission warnings.
gcc/
	* doc/invoke.texi (-Wdeprecated-variadic-comma-omission): Document.
gcc/testsuite/
	* g++.dg/cpp26/variadic-comma1.C: New test.
	* g++.dg/cpp26/variadic-comma2.C: New test.
	* g++.dg/cpp26/variadic-comma3.C: New test.
	* g++.dg/cpp26/variadic-comma4.C: New test.
	* g++.dg/cpp26/variadic-comma5.C: New test.
	* g++.dg/cpp1z/fold10.C: Expect a warning for C++26.
	* g++.dg/ext/attrib33.C: Likewise.
	* g++.dg/cpp1y/lambda-generic-variadic19.C: Likewise.
	* g++.dg/cpp2a/lambda-generic10.C: Likewise.
	* g++.dg/cpp0x/lambda/lambda-const3.C: Likewise.
	* g++.dg/cpp0x/variadic164.C: Likewise.
	* g++.dg/cpp0x/variadic17.C: Likewise.
	* g++.dg/cpp0x/udlit-args-neg.C: Likewise.
	* g++.dg/cpp0x/variadic28.C: Likewise.
	* g++.dg/cpp0x/gen-attrs-33.C: Likewise.
	* g++.dg/cpp23/explicit-obj-diagnostics3.C: Likewise.
	* g++.old-deja/g++.law/operators15.C: Likewise.
	* g++.old-deja/g++.mike/p811.C: Likewise.
	* g++.old-deja/g++.mike/p12306.C (printf): Add , before ... .
	* g++.dg/analyzer/fd-bind-pr107783.C (bind): Likewise.
	* g++.dg/cpp0x/vt-65790.C (printf): Likewise.
libstdc++-v3/
	* include/std/functional (_Bind_check_arity): Add , before ... .
	* include/bits/refwrap.h (_Mem_fn_traits, _Weak_result_type_impl):
	Likewise.
	* include/tr1/type_traits (is_function): Likewise.