Add support for -std=c++2a.

Add support for -std=c++2a.

	* c-common.h (cxx_dialect): Add cxx2a as a dialect.
	* opt.c: Add options for -std=c++2a and -std=gnu++2a.
	* c-opts.c (set_std_cxx2a): New.
	(c_common_handle_option): Set options when -std=c++2a is enabled.
	(c_common_post_options): Adjust comments.
	(set_std_cxx14, set_std_cxx17): Likewise.

	* doc/cpp.texi (__cplusplus): Document value for -std=c++2a
	or -std=gnu+2a.
	* doc/invoke.texi: Document -std=c++2a and -std=gnu++2a.

	* lib/target-supports.exp (check_effective_target_c++17): Return
	1 also if check_effective_target_c++2a.
	(check_effective_target_c++17_down): New.
	(check_effective_target_c++2a_only): New.
	(check_effective_target_c++2a): New.
	* g++.dg/cpp2a/cplusplus.C: New.

	* include/cpplib.h (c_lang): Add CXX2A and GNUCXX2A.
	* init.c (lang_defaults): Add rows for CXX2A and GNUCXX2A.
	(cpp_init_builtins): Set __cplusplus to 201709L for C++2a.

Co-Authored-By: Jakub Jelinek <jakub@redhat.com>
From-SVN: r252850