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, 1 insertions, 1 deletions

diff --git a/libcpp/include/cpplib.h b/libcpp/include/cpplib.h
index d326f5a..aef703f 100644
--- a/libcpp/include/cpplib.h
+++ b/libcpp/include/cpplib.h
@@ -174,7 +174,7 @@ enum c_lang {CLK_GNUC89 = 0, CLK_GNUC99, CLK_GNUC11, CLK_GNUC17, CLK_GNUC2X,
 	     CLK_GNUCXX, CLK_CXX98, CLK_GNUCXX11, CLK_CXX11,
 	     CLK_GNUCXX14, CLK_CXX14, CLK_GNUCXX17, CLK_CXX17,
 	     CLK_GNUCXX20, CLK_CXX20, CLK_GNUCXX23, CLK_CXX23,
-	     CLK_ASM};
+	     CLK_GNUCXX26, CLK_CXX26, CLK_ASM};
 
 /* Payload of a NUMBER, STRING, CHAR or COMMENT token.  */
 struct GTY(()) cpp_string {