c++: Add support for -std={c,gnu}++2{c,6}

It seems prudent to add C++26 now that the first C++26 papers have been
approved.  I followed commit r11-6920 as well as r8-3237.

Since C++23 is essentially finished and its __cplusplus value has
settled to 202302L, I've updated cpp_init_builtins and marked
-std=c++2b Undocumented and made -std=c++23 no longer Undocumented.

As for __cplusplus, I've chosen 202400L:

  $ xg++ -std=c++26 -dM -E -x c++ - < /dev/null | grep cplusplus
  #define __cplusplus 202400L

I've verified the patch with a simple test, exercising the new
directives.  Don't forget to update your GXX_TESTSUITE_STDS!

This patch does not add -Wc++26-extensions.

gcc/c-family/ChangeLog:

	* c-common.h (cxx_dialect): Add cxx26 as a dialect.
	* c-opts.cc (set_std_cxx26): New.
	(c_common_handle_option): Set options when -std={c,gnu}++2{c,6} is
	enabled.
	(c_common_post_options): Adjust comments.
	* c.opt: Add options for -std=c++26, std=c++2c, -std=gnu++26,
	and -std=gnu++2c.
	(std=c++2b): Mark as Undocumented.
	(std=c++23): No longer Undocumented.

gcc/ChangeLog:

	* doc/cpp.texi (__cplusplus): Document value for -std=c++26 and
	-std=gnu++26.  Document that for C++23, its value is 202302L.
	* doc/invoke.texi: Document -std=c++26 and -std=gnu++26.
	* dwarf2out.cc (highest_c_language): Handle GNU C++26.
	(gen_compile_unit_die): Likewise.

libcpp/ChangeLog:

	* include/cpplib.h (c_lang): Add CXX26 and GNUCXX26.
	* init.cc (lang_defaults): Add rows for CXX26 and GNUCXX26.
	(cpp_init_builtins): Set __cplusplus to 202400L for C++26.
	Set __cplusplus to 202302L for C++23.

gcc/testsuite/ChangeLog:

	* lib/target-supports.exp (check_effective_target_c++23): Return
	1 also if check_effective_target_c++26.
	(check_effective_target_c++23_down): New.
	(check_effective_target_c++26_only): New.
	(check_effective_target_c++26): New.
	* g++.dg/cpp23/cplusplus.C: Adjust expected value.
	* g++.dg/cpp26/cplusplus.C: New test.

1 files changed, 4 insertions, 3 deletions

diff --git a/gcc/doc/cpp.texi b/gcc/doc/cpp.texi
index b0a2ce3..3f492b3 100644
--- a/gcc/doc/cpp.texi
+++ b/gcc/doc/cpp.texi
@@ -1908,9 +1908,10 @@ selected, the value of the macro is
 @code{201402L} for the 2014 C++ standard,
 @code{201703L} for the 2017 C++ standard,
 @code{202002L} for the 2020 C++ standard,
-or an unspecified value strictly larger than @code{202002L} for the
-experimental languages enabled by @option{-std=c++23} and
-@option{-std=gnu++23}.
+@code{202302L} for the 2023 C++ standard,
+or an unspecified value strictly larger than @code{202302L} for the
+experimental languages enabled by @option{-std=c++26} and
+@option{-std=gnu++26}.
 
 @item __OBJC__
 This macro is defined, with value 1, when the Objective-C compiler is in