c++: Add support for -std=c++23

c++: Add support for -std=c++23

Derived from the changes that added C++2a support in 2017.
r8-3237-g026a79f70cf33f836ea5275eda72d4870a3041e5

No C++23 features are added here.
Use of -std=c++23 sets __cplusplus to 202100L.

$ g++ -std=c++23 -dM -E -x c++ - < /dev/null | grep cplusplus
 #define __cplusplus 202100L

gcc/
	* doc/cpp.texi (__cplusplus): Document value for -std=c++23
	or -std=gnu++23.
	* doc/invoke.texi: Document -std=c++23 and -std=gnu++23.
	* dwarf2out.c (highest_c_language): Recognise C++20 and C++23.
	(gen_compile_unit_die): Recognise C++23.

gcc/c-family/
	* c-common.h (cxx_dialect): Add cxx23 as a dialect.
	* c.opt: Add options for -std=c++23, std=c++2b, -std=gnu++23
	and -std=gnu++2b
	* c-opts.c (set_std_cxx23): New.
	(c_common_handle_option): Set options when -std=c++23 is enabled.
	(c_common_post_options): Adjust comments.
	(set_std_cxx20): Likewise.

gcc/testsuite/
	* lib/target-supports.exp (check_effective_target_c++2a):
	Check for C++2a or C++23.
	(check_effective_target_c++20_down): New.
	(check_effective_target_c++23_only): New.
	(check_effective_target_c++23): New.
	* g++.dg/cpp23/cplusplus.C: New.

libcpp/
	* include/cpplib.h (c_lang): Add CXX23 and GNUCXX23.
	* init.c (lang_defaults): Add rows for CXX23 and GNUCXX23.
	(cpp_init_builtins): Set __cplusplus to 202100L for C++23.